1. I'm pretty sure everyone is sick of it, the NPC's change voices. Like when you talk to a beggar she uses smooth voice to speak unique conversation with you , and then when saying good bye she's back to the scratched voice. Even NV still has this problem, so we can't just blame on voice acting being "new". Four years have passed and still we have this issue.
2. NPCs can't seem to ignore you while they ignore others. Have you ever followed an NPC in a quest, and if you are too close he'll turn around and say hello to you, and stand for a few seconds before moving on? Very annoying. The NPC's should have "chatty mood" and "quiet mood" at different times, and they should ignore you at random times.
3. American accent. In FO it is ok, but in Oblivion, is everyone in Cyrodiil American? Heck even in America people have different accents. In Oblivion we lost the Morrowind-style dark elf voice, and we need it back this time. Also, different races should have different accent, such as bretons with british accent, nords with either Slavic or Scandinavian, and such. By the way, I do recognize that the dummers in Morrowind have that voice is due to the volcanic ash there.
4. Not about voice, but the gestures and body language. The only body language I have seen so far is the "put the hand on the hip" thing. We definitely need realistic and DIVERSE body language.