Diesel - agreed that it's a big bite of the apple. But everyone makes it. It's just harder, and more confusing(yes, confusing), getting through the school. Payoff is some interesting flying. I still remember my first 767 FO trip to Europe - internal grinning from ear to ear for hours. Flying into the Caribbean for the first time, the list goes on and on vs. previous flying. And when you go back to the 737 you'll think "man, this is kinda little..."
Almost everyone is going to a different airplane then they're currently flying. Learning curve for everyone. Some steeper, some flatter. Any prior Boeing experience makes the 737 or 757/767 school easier(FMC/instruments/packs/bleed systems similar).