The big 3 in my book are:
The slide stop re-insertion, this should be done without the recoil spring installed. After slipping the pin in, let it dangle straight down then push and pull the barrel to make sure you captured the link. Part B of the pitfall involves the "idiot mark". You've inserted the pin, the slide stop is dangling down around the trigger and you slide the pin out enough to avoid scratching the frame as you pivot it in to place. Once aligned with the frame notch, pinch with the left thumb and forefinger to control it and apply up and in pressure with the right thumb. If it does not slip in easily, using full control with your hands, position it just a tad lower on the plunger and retry.
The sear spring. Remember to place it on top of the sear not under.
The magazine catch lock. Another part that gets buggered up easily. Don't force it, it will turn when it's aligned and not a mm sooner.
A few other hints:
When removing the thumb safety, cup your right hand around the rear tangs to catch the spring (if not crimped).
Point the spring plug at your hand not your face.
Engage the thumb safety when re-installing the slide.
After laying the sear spring in place, slide the MSH on to hold it in place.
Don't go crazy with the oil and make sure to remove any on the breechface and feed ramp.
Take your time and keep some cheap Tupperware around for those parts. Good luck and remember it only gets easier.