Better voices for males as someone already mentioned. The main thing though, for me anyway, is body changes. It breaks my immersion to see Halsin as an elf flexing around camp. It also bothers me that fighters and probably barbarians would be the same size as a wizards and other pure spellcasters. Half casters I could understand but come on.
I could guess it has to do with mesh codes, but whatever. Let my martial classes actually look like they can fight in melee.
Look at Githyanki as a warrior and you will see how stupidly he looks in armor.