The cockpit is adequately sized - If you are a Hobbit. Seriously though, as you mention the 737 is pretty good at all of the "important stuff." Its reliable, has good displays and tight NAV, nice capabilities with the HUD etc - its the little things that Boeing cheaped out that drive me crazy:

Seats recline or move aft but can not do both at the same time.
The heating and cooling system was designed by Rube Goldberg himself.
The sun-shields are a joke - I really can't believe this is their serious attempt to block the sun from Pilots.
Their is a spar below the jumpseat that will literally tear your bag to shreds if you lay it flat - you know, the way everyone would normally store your bag.
The two prongs that "catch" the Jumpseat as its lowered are conical and very sharp - much better to rip the FOs pants as gets in and out - why not make them rounded?
Speaking of getting in and out you have to be gymnast to get in and out of the seats. The 90 degree turn in the seat track that always jams is very effective in adding to this challenge.
Even Hobbits complain about the size of the Front Lav.
Cupholders that fall out and make a Chrysler Minivan look luxurious.
Oh Yeah we cant forget about the seat cushions that help put many chiropractors kids through college. Now to be fair this could be a Delta issue for not replacing the padding and cushioning as needed. Some of these things are so worn and compressed you would be better served sitting on a piece of plywood - except that now costs more than a new cushion.
Its not just the inflight noise that is a pain turning on the packs and recirc fans at the gate is a very good simulation of a hurricane. There is a reason the "Two guys in a phonebooth during a hurricane" meme sticks around.

The antique overhead doesn't even bother me becasue it works - plenty of other things in the 737 do not.

Scoop