PvP: Top arena teams are almost always consisted of Alliance players, and if you are one of those faggots who think that BG's take more skill then arena, most battle groups are balanced in terms of Alliance vs Horde win loss ratios. In actuality the horde dominates just as many battlegroups as the alliance. But hordies love to brag, and thus are "L33T"
PvE: As for PvE the number one alliance guild is only a hair behind the number one horde guild. But your chances of getting into ANY of the top guilds in the world is slim to none, unless you cast away that useless thing you call a life into the gutter. In PvE, Alliance vs Horde progression is completely dependent on your server.
Personality: There is a common misconception that all Alliance players are 13 years old, but those people make up a small portion of the alliance. Although the horde is generally more mature then the alliance, that isn't to say that you aren't going to run into your fair share of retards. People on the horde tend to believe they are the best at everything, and never take anyones advice, regardless of how good it is.
If you are a girl you will be hit on/molested constantly.
Random message board/youtube video featuring alliance players.
Alliance Player: Cool video/post! I learned a lot, can you tell me what addons your using?
Poster: @coolguy11 Sure its blah, blah, blah
Alliance Player: Awesome thanks!
Faggot: THE HORDE IS SO FUCKING AWESOME ALLIANCE SUCK! FOR THE HORDE MY LEVEL 85 BLOOD ELF DEATH KNIGHT NAMED BLOODDEATHSHADOW WILL PWN YOURE ASS ANY DAY1!!!11 LOL!
Poster: @Inuasha1 dude chill
ArrogentProGAYmer: You are an idiot and will never be good, noob
Poster: @eliteshaman31 umm okay...
Dude1: Dude did you hear?
Dude2: No what?
Dude1: Paragon just downed (boss x)!!!
Dude1: SO THEIR TOTALLY THE BEST IN THE WORLD!!
Dude2: Yeah... best at PvE.... thats like being the best at call of duty.
Alliance Guy: I would take annoying kids over basement dwelling, mouth breathing, neck beards any day of the week.
Horde Guy: *alt tabs out of 4chan and/or puts down hentai* Yeah but you guys fucking suck