I realy dislike:
Bad paintjobs, if you're going to do it, do it right!
just about anything premod
people that "rice" their case without having preformance in mind (why waste $$$ if you're not going to make it go faster?).
biohazard, atomic, spiderman, etc. It was cool when it first came out, but it is old news. Get with it!!
"perfect wiring" I honestly don't care about how my wiring looks, as long as it doesn't impeed air-flow. Sure, if I do a mod I'm going to tidy my cables, but I'm not going to fold my IDE cables into some origomi shape to make it look neat!!
windows to show off, a stock computer. Guys, the only reasons you should even have a window is if you have something to show off!!!!
Acrylic cases, possibly this is just me, but I don't peticularly like most acrylic cases. Reason: you have to have everything perfectly arranged in a acrylic case! There is nowhere to hide!!!
LCDs with no purpose, if you have an LCD, USE IT!!!!
Oh, one more, Hard drive windows on NEW drives. Sure, they are awesome, but don't do it to a drive you plan on actualy using for anything marginaly important! Sheesh
