Unless you have a strong preference, one of the best ways to choose a race for your character is to match the race of the existing DP character you'd most like to play with. Even if that character is level 49 already, chances are that player will take the time to roleplay with a brand-new member of their own race. It makes sense, IC, for fantasy characters to "take care of their own" and serve as mentors, companions, friends, and lovers to those of their own race.
A note about racism: DP of course condemns real-life racism in all its forms. In fact, there are no separate human "races." Biologically and scientifically, humans are actually one race with variation in coloring and facial features.
Not so for the different "races" in fantasy worlds. Racial tensions are one of the primary sources of story conflict in fantasy worlds, and DP fights the tendency to "bleed" cultures together and play all races the same way. Variety is the spice of life. If you want to act like a dwarf, play a dwarf! If you want to act like a troll, play a troll! Don't let superficial concerns (I don't like the way the dwarf models look/talk) stop you from being true to the game world as presented... use your imagination to rise above the limits of game graphics and imagine your character to look however you like. (We even have a thread on our message board for that!).
Azeroth's player races are divided into two affiliations, the Alliance and the Horde, with five races representing each. A simple way of thinking of Alliance vs. Horde is "humans vs. orcs." The humans are the central and most populous race of the Alliance, and the orcs serve the same function for the Horde. It is important to know, however, that although the only two truly evil races (trolls and undead) do belong to the Horde, the Horde itself is not evil. Either affiliation is valid for players who want to tell stories about heroic "good guys," and either affiliation can contain quite reprehensible villains as well.