Well, there's always the obvious one: they carry and they AREN'T an asshole about it.
But other things can catch my eye too. I've honored people who have saved me from stupid deaths. Or people who have run stupid chat gags with me over the course of the game. (One of the reasons I wish they'd bring back Honorable Opponent.)
Hell, I've honored people who, despite having been harassed all game by another teammate, just kept playing like it was no big deal. Because that takes a lot of self control not to fire back while being flamed.
TLDR: I look for people who stand out amongst the rest. Usually, my decision is based off attitude before gameplay though.