Okay just my take on things ... tweak it if you must
Beast Races- They are beasts for a reason and need to look that way! Make them look fierce and make them walk the way they were made to, not like humans! I'm hoping to see a lot of good customizations for them as in manes and horns and stuff like that!
Human Races- In my opinion the human races were pretty good in OB but lacks variety. Like most people have said they were hard to tell apart aside from the voices but even then I still got them confused. Personally I'm not to big on the human races so I wouldn't know how to tweak the details to make them different.
Orcs- Kinda self explanatory if you ask me. Give them options to look like hell spawn demons! They looked too kind in OB and that kinda pissed me off. Make them raged and bigger, preferably a deeper voice and less human quality's!
Elven Races- Like people have said give them differences in the ears and give them natural set eyes! The high elves should look majestic, Dark elves need to look sort of evil and less radiant, Wood elves should be nimble.
So please correct me where I'm wrong because I'm sure there's some things I left out but this is just a basic lay out.
Dunmer are in no way evil, but their appearance needs to be closer to the one seen in Morrowind if you ask me, gray skin tones, red eyes (Red irises that is, all red
will look awful, I can guarantee that.) and their hair and facial hair needs to have a distinct look. What I liked about the Dunmer in Morrowind was that they truly felt like their own people, in Oblivion they were just elves with darker skin, that's not what the Dunmer are, they are vastly different as a people form the other elven races.