I'll 10th or 11th or whatever the paladin/monk/all multiclass restrictions. If the DM wants to restrict a PC, that's one thing, but there's no reason to put it in the rules as a default. Pretty much any other rule I can see some kind of rationale for. Even death from massive damage... sure, if you can take 50 damage you'll usually pass the DC 15 save, but it at least is trying to model some kind of standard "death from extreme shock" that makes sense in theory, even if the exact implementation is awful.
For a truly superfluous rule -
From Settlers of Catan (4 player game, each player has his own color pieces, color means nothing except whose pieces are whose)
"If you're playing with only 3 players, don't use the red pieces."
-Nate