The coup de grace rules. Perhaps I misread them, but if CdG is a standard action, then an opponent can go all the way across the battlefield and end someone. In my book, any intelligent creature will CdG opponents, for two reasons: (1) Each downed opponent has a 5% chance per round of getting back up; (2) If the opposing side has any healers, the opponent will *certainly* be back up.
Now, I know the DMG advises DMs against having opponents hit players when they're down, and that advice sticks in my craw. It's kind of like the old Paladin Problem -- balancing a rules effect with a roleplay effect. Paladins get X, but are restricted by moral code Y.
At heart, that *is* what bothers me. Rules crunch is, in 4E, balanced by DMG fluff. Everything from wish lists (rules crunch economy balanced by DMG fluff Santa Claus) to the coup de grace rules.
So, to encapsulate my "proud nail": I don't like rules that are only balanced if I, as DM, play the way that the (rather amateurish, to be honest) DMG wants me to play. I would have loved the 4E DMG at age 15, and I think the 4E DMG may be just the tool to teach a new generation how to do it, but really, after DMing for 20 years I think I can take the training wheels off.