Aside from rules lawyers, what really irks me is when someone new comes into play just for the night. Like one of our runners brings a friend along for one ride. Said friend wasn't even in our main group, just someone we(my group and I) occasionally hung out with, and he brings his friend (whom none of us knew) and who didn't know the game... So he tried to explain things to his friend, and we were just..... No...
It's annoying when people do things uninvited that mess with the Mojo of the whole group.
The building a character for mainly stats... I find it acceptable to an extent. You have to make a backstory. I made a character (2nd edition... All I play now, I have 4th ed core book, reading it...) who was a beast with a bow. I'm talking long range shot, slotting a troll in heavy armor. A beast, but he had a backstory....