I think Caden summed it up pretty well. I totally agree. I played horde from the start and i made a lot of friends who would help you if you asked or would make a peice of gear for you even if you couldn't pay him/her enough. Then i started to play alliance (mostly cause my friend whom i played with on Horde went to college) I started a night elf but the people seemed, well... uptight. I also think the Horde races have better racial abilities.
Undead: Will of the forsaken, this spell is awsome for PVP. you get feared or traped or sheeped, anything that maikes you lose control of your character, cast this little racial ability and you are free once again. Cooldown is farely short, i think like 2 mins. Also you can hold your breath under water 300% longer than all the other races. Not to mention you can eat dead corpses to restore your life (humanoid or undead corpses)
Trolls: regen health even during combat. This makes good for mages, warriors, or any other troll class that can't heal themselves. This reduces downtime and might even save your life every once in a blue moon. they also have berzerk which i belive increases attack speed for a short period of time, this is excellent for the mellee classes.
Tauren: they have warstomp that stuns up to 5 enemies at a time for 2 sec which for the healing classes is enough time to get a good heal off or escape.
Orcs: I can't remember what they have, but who needs extra abilities anyways when your mean green and full or rage?
Personally i like the areas that horde prosper in because it actually has the feel to being on a chaos stricken planet that is at constant war with eachother and i believe you get more of a feel for the history playing horde as well. Alliance players IMO just goof off in goldshire and stur up trouble. Horde seems to be at a state of harmony but is very professional when it comes to taking care of buisness and takeing care of eachother. This goes for NPCs and player alike.
There are other racial abilities for the classes that help a lot but I can't remember them all. These are just the ones that stick in my mind.
Hope this helps.