What you'll take for granted? The list is nearly endless. You'll take for granted what you don't know, and you won't know what you don't know.
Navigation. Today's pilot breed are children of the magenta line. That means snot nosed kids who don't know anything more about finding a destination than following the little colored line on their GPS, and I see a lot of it. Don't be one of those. It wasn't that many years removed when such stuff was not found in most cockpits, and we all flew with paper maps first, before acquiring other things in the cockpit. Understanding how to navigate without crutches is important before tackling automation, because if you wait, you'll likely never go back to properly learn the fundamentals.
Pilots today are shocked at the concept of executing a forced landing off-airport. There was a time, not so far removed, when a foundation of elementary flight training was being able to put the airplane safely back on the ground in the event of a power failure. Today it's hardly taught, given a slight wave by instructors who have never experienced it themselves, and aircraft are had with parachutes that are treated like alternate airports. Pilots blast off into conditions they should not, in light piston powered airplanes at night and in bad weather, with the misguided notion that the it's okay because the parachute is there to save them. They take this false sense of security for granted and think nothing of it.
I was recently in a simulator when a grey-haired instructor asked about a reciprocal (opposite heading), and I saw someone with the familiar deer and headlights gaze. It's second nature if someone understands the concept of flying a non-directional beacon, but foreign to someone who does nothing but fly the magenta line. Understanding fundamentals, even for things that aren't used much today, is still foundational for flying skills and may save your life. Don't take today's modern technology for granted.
Included in that technology is ADS-B and TIS, features you'll learn about which, among other things, aid in avoiding collisions with traffic. FAR, FAR too often, I hear traffic reported by air traffic control, and pilots respond with the second most idiotic thing they can think of, which is "got 'em on the fish finder." Don't let your equipment look for traffic on your behalf. You look. Look out that window and scan aggressively like your life depends on it, because it does. So does mine.
I hear far too many pilots who reach the departure end of the runway, while taxiing, and announce in their best cigar-choked drawl, "Shouldawouldacoulda Four Five Niner departing runway Three Three. Any inbound traffic please advise." They're taking something for granted; not everyone is listening, not everyone is reporting, and not everyone is replying, and nobody is required to do any of the above. Letting one's radio look for traffic is among the most stupid things a pilot can do, save for putting too much air in the fuel tanks before departure.
Speaking of air, it's said (rightfully so) that air above you on takeoff does you very little good, as does air behind you on the runway, and most of all, air in your tanks. Best to replace it with fuel before you go fly, and be conservative. Carry lots. Especially in light airplanes. Don't trust gauges. Or instructors who are wet enough behind the ears that they've never seen or experienced and engine failure, a fatality, or a real emergency. That's most of them. Don't get too pie-eyed about the instructors, either; they may seem like authorities, but most are no-experience, no-flight time know-nothings with fresh, wet commercial certificates who were just recently student pilots themselves; you're receiving training in most cases from the absolute lowest common denominator in the industry. Keep that firmly in mind before you let that person kill you.
There was a thread here not so long ago by an individual whose instructor took on a night cross country flight, and struck a deer. The instructor insisted it was okay to fly the airplane home, which is definitely not the case. It wasn't legal or safe. Trust, but verify, and then forget trust. Just verify. There are a lot of ways to get hurt or killed in aviation: the trick is to avoid discovering another. Or celebrating one of the many knowns that keep happening.
It's trite, but keep your airspeed up. Especially close to the ground, such as turning to final approach. Avoid get-there-itis, the inherent need to complete a flight or maneuver in spite of weather, fuel, mechanical problems, health issues, etc. There is no flight which must be made. None. I've flown time-critical organ recoveries with hearts and kidneys, combat zones, fire and law enforcement, and all kinds of other duties, utilities, and missions, and I have yet to see or hear tell of any flight which MUST be made. Don't take for granted the word "no." It's short, easily overlooked, and one forsaken by many. Your job, when you arrive at the airport, is to look critically for any excuse to say "no." Only if you can't find one after an exhaustive search, should you fly, and then keep it in mind at all times.
Learn the maintenance aspects of what you fly. MOST pilots take this for granted, and most probably couldn't tell you squat about what they're seeing when they do a pre-flight inspection on an aircraft. Know. Your life depends on it. Should that fiberlock nut be there? Ask the dead P-51 driver at Reno how many times on can be used, and where it should be used...because the wrong use of the wrong fastener not only killed him, but burned up a grandstand full of spectators. One little nut. How many threads must protrude past the end of the nut? Pilots look, but 9.99 out of 10 couldn't tell you, and that's exactly the kind of thing, taken for granted, that can kill you. Know. I was gently encouraged to read the bible on aircraft maintenance, AC 43.13, when I was a student. Encouraged might be underselling. It was crammed down my throat until I bled, and I did learn it, and I am alive because of it...many times over. Don't take anything for granted.
The fueler knows what he's doing. He wouldn't put Jet-A fuel in your avgas piston Cessna. Or would he? Don't take that for granted. The weather channel said everything would probably be okay, no need to check and update weather. Or is there? It's just over the counter medication. They wouldn't sell it if it posed a danger. Or would they? It's just one beer...you get the idea. The mechanic who did the engine overhaul said everything was okay. The oil under the cowling, on the nose strut, is just some that someone spilled right? I wouldn't take that for granted. I wouldn't take for granted that the brakes are safety wired correctly. Or that the work was done correctly, or at all...check everything you can. Open, probe, touch, check.
As a CAP cadet, I was called for a search mission that was urgent. It was close to the end of the day and they wanted an airplane in the air fast. Three of us showed up, pulled the airplane out. I walked around the back and the elevators were missing...the parts that control the up and down. Actually gone from the aircraft. Removed. Take nothing for granted.
I returned from a mission in Iraq one night and parked; the aircraft needed fuel. I ran to the dining facility on base before it closed, and when I came back to check on the aircraft, it was gone. The individual who took it got airborne before apparently discovering no fuel, and when he got on the ground, he was irate that I hadn't ensured it fueled. I asked why he elected to take an aircraft with no fuel on board and he said he didn't check, that he trusted everyone to ensure it was fueled after a mission. How much traction that get him? Take nothing for granted.
Graveyards are full of people who didn't look for traffic, who didn't calculate performance, who assumed they had the instrument flying skills that they lacked, who trusted the other guy in the cockpit a bit too much. Eastern Airlines put an airplane in a swamp while three cockpit crewmember tried to troubleshoot a lightbulb. A mechanic assumed oxygen generators were safe and the crew trusted the mechanic; the aircraft caught fire and dove into the swamp outside Miami, killing everyone. A first officer (copilot) didn't speak up on a takeoff in Washington DC and rode the airplane into an icy river, even though he knew the airplane wasn't producing power. Cooperation is one thing, but blind faith in instruments, air traffic control, other crew, mechanics, management, aircraft manufacturers, the FAA, or anything else, is very dangerous. Flying should be fun, but safe. It can't be if anything is taken for granted.
I was working in a shop when several new, low time pilots bumbled in. We asked who preflighted the airplane. One proudly jabbed his thumb at his chest and claimed he did. I asked him about his tire pressure. He checked it with a calibrated pressure gauge, he said. How, I wondered aloud; the gauge is off being calibrated, and nobody asked for mine, out of my tool box. I dug it out. Check the pressure. 50% of what it should have been. A recipe for a blown tire, damaged wheel, loss of control, even hydroplaning, uneven braking, control issues, excess tire wear, etc The pilot admitted he'd just looked, assumed it was right, and used his "calibrated eyes." Don't assume. Don't take for granted.
I took a parachute for granted many years ago, but woke up in intensive care on life support, a big memory gap, a lot of stitches, and an unbelievable amount of pain. I'd made some assumptions along the way, and like the chain of events that leads to most mishaps (there are no accidents), I failed to prevent what nearly killed me. Don't assume. Know.
Pilots don't like to train in severely underpowered airplanes these days. You don't see people rushing to find Luscombs and Cessna 150's and Cubs to fly on hot days...they like performance. Performance hids judgement errors and bad decisions. Comfortable, easy-to-fly airplanes hide bad habits; nosewheels mask landing errors that tailwheels would never permit. I don't see pilots forcing themselves to land on the numbers much these days. Don't take for granted. Don't let power or performance or instrumentation lull you into complacency.
The military insists on aerobatic training, makes it's pilots explore the flight envelope. Civilian training doesn't, and most pilots never bother on their own. Today many enter the pipeline to learn and earn the bare minimum, rush to a regional airline cockpit, and therein lies the extent of their effort for the remainder of their career. They don't know what they don't know. I see far too many who think the industry and their career should be handed to them on a silver platter, learning and training injected by osmosis. Learn, study, sweat. Treat your flight training like your Phd studies. Your training airplane may fly low and slow, but can kill you just as dead as a high speed, low drag fast moving piece of flashtrash; it doesn't matter what you're flying whether it's an F22 or a lowly Piper Cub; respect it, learn it, and don't take it for granted. Tens of thousands died to form the bedrock of the regulation, safety standards, maintenance and flying practices that are in use today, and yet we continue to see people make the same mistakes with controlled flight into terrain (running into mountains and trees and powerlines), and fuel exhaustion. Don't take for granted the deaths and lessons that came before; you'll never meet them, probably never read about most, but the lessons are as cogent today as ever.
Remember that it's the traffic in flight that you don't see that kills you.