For me
-Oblivion like levels, the first three we're okay, but after that they just got tedious and boring. I hated the allies for bruma quest.
-Being able to "wait" and have your health and mana recharge. I could understand in MW when it was "rest" and you couldn't do it in a town, but I think "waiting" shouldn't do anything but make the time go faster.
-Invisibility spells. You were basically a god if you could cast one. There was no need for the sneak mechanic, and it made it way to easy to get out of fights. I do want chameleon, but only up to about 65%.
-Lockpicking mini-game. With this there is no reason to have the security skill.