A lot of the D&Disms I despise have been removed in 4e but here they are:
Save or Die/Lose - I have a hard enough time keeping my players attention on the game, I don't need game mechanics that force them to sit out and go play Nintendo because they failed a save and won't possibly be able to recover until combat ends 1 or 2 hours from now.
Level Drain - You spent hours working up your character only to lose it all because a monster touched you? Are you freaking kidding me? Possibly one of the worst ideas in the history of gaming. Sorry, Gary. But its true.
Excessively fragile 1st level characters - Yay! I made a character and I'm ready to play! ....Only to die 5 minutes later in the first round of combat.
Vancian casting/X per day slots/powers/abilities - I dislike slots and any sort of per day game mechanic. It bugs me in 4e too.
Magic item dependency - Non-spellcasters were really defined by their items because it was really the only way to gain an ability that wasn't just swinging their sword and rolling damage. And it was also the only way they could begin to keep up with spellcasters at high level.
Alignments - I don't necessarily mind alignments but spells like Detect Evil were annoying as a DM.
Lawful Stupid Paladins - Sorry, but I view paladins as zealous holy warriors bringing fire and death to the enemies of the faith. Not social workers who get stripped of their paladinhood because they didn't read the monster its Miranda rights before they smote it. "Bring the villain to justice" can mean cutting the bad guy in half. And the only "legitimate authority" a paladin needs to recognize is their god and church. Keep your modern notions of jurisprudence out of my fantasy game.
D&Dism that I actually like:
HPs - I hate death spiral systems where the PCs get increasingly penalized as they get hurt. D&D HPs may not be realistic but its fun.
Leveling - Something fun about leveling up your PC and choosing new abilities.