I'm not even really done my PC which is in my sig; I won't consider it complete until my GPU is replaced, but I have learned a lot, not only from you guys, but from doing it myself. Mostly learning things the hard way, luckily not the fatal way:
-I don't like LEDs, and I don't like flashy cases. Next case will be a black box, next fans will have no LEDs.
-Rushing and buying parts quickly just so I could get on with replacing OEM stuff was a waste of money, patience is a virtue. There was no need for me to grab an H80i or a 4670K over what was recommended, but I did it anyway because I was so anxious to just finish my PC Part Picker checklist and get on with the build. Didn't need to rush out and pay more for those parts at all, and in the H80's case, I already had a EVO 212 on the way...so, dumb decision.
-It's not all about the parts; The right tools for the job help out a lot. Relying on an old chewed up screwdriver will annoy you more than that stock Intel cooler you installed "just to see what it's like." After a day of assembly I went to the local Part Source and bought a $15 tiny Phillips screwdriver and thanked myself for that purchase every time I've had to use it. I've also bought a flashlight and a couple adjustable lamps for those hard to see places.
-It's not as easy as people say it is, but that's mostly because it's your first build and you're nervous about doing it all correctly the first time. Once the jitters go away and you see that boot screen, once your thread that translates roughly into I FUCKED UP HELP WAT DO" gets some replies, it's smooth sailing. Mostly.
-Label and segregate those screws when you're disassembling! One knee to the table as you're getting up and now you don't know what goes where, or why you have those two extra screws. when you just put the side panel back on.
-Static is scary. The first time I picked up my 7850 and shocked myself, I got pretty scared. Lucky for me nothing went wrong.
How about you guys? What lessons did you learn from your first build?