More races = more armours (so that's less armours for human characters?). I don't see ANet make 3 or more sets of the same armour for all those different races (if they have male/female as well that's even twice as many).
On the other hand, I'd prefer more races over more professions, since I'm not at all satisfied with the current profession balances.
IF there would be more races however (I don't really care) it should be existing ones, especially dwarves (not that I would play one but they fit).
No "bad" races like devourers, Charr, undead, mursaat, blood drinkers or whatever.
But for variety, I'd rather have far more armour, faces, hairstyles, white dyes, hairdressers, thin/normal/fat characters, young/old characters than different races.