Well people always ask me why I like Horde over Alliance. I actually like the Alliance races better and was expecting not to like the Horde before I tried them. I have played both for about 6 months each on a few different servers and these are my observations.
1. When putting things for sale in the trade channel, Alliance players love to correct your pricing and make fun of the item you are selling. Horde don't give a shit they wont care if you try to sell a dig rat for 100g.
2. When selling things Alliance players always try to haggle you down and make excuses why they don't have the money. Horde just pay.
3. Alliance players expect you to be best friends for life if you help them with a quest. Horde team up get the job done and get say thanks and leave.
4. Alliance player are very cowardly, they will only attack if they have more people or vastly higher levels. Horde players are braver and love a good fight.
5. Alliance players are clueless when it comes to Battle Grounds. Horde players actually listen to what the leader is saying and have some kind of unity thats why they win 9 out of 10 times.
6. Alliance players are not trust worthy and will stab you in the back if you make "friends". For some reason Horde are usually more honorable.
7. Alliance players will report you for asking to buy a walk threw a dungeon form a higher lvl on trade channel. Horde realize this as a valid transaction and don't bother you.
8. The Horde Trade and general chat is way more funny and interesting than the Alliance.
9. Horde can say "FOR THE HORDE" while going into battle or when he knows he will die. If Alliance say "FOR THE ALLIANCE" it's just gay.
Of course this is all only for the most part. There are always the occasional times where it's the opposite but these are my true feeling on the issue.
I'd say the trade channel is the major one for me. I don't like hearing little punk know it alls making fun of my asking price and whispering me saying I'm stupid.
Now FOR THE HORDE