| Nobody has mentioned it, so I will.
If you havn't noticed some servers are <PVP>, some are <Normal>, some are <RP> and some are <RPPVP>; at least on the US servers. Now I'm no expert, but I believe the way it works is this. Normal servers (the one I play on) you must flag for PvP, or attack someone flagged for PvP. You can flag manually by clicking on your character bar. PvP servers it's free game for anyone at any time. I think RP servers are the same as Normal servers, just with a focus on Roleplaying by the players. And I believe that RPPVP servers are the same as PvP servers, just with the same focus on RP as the RP servers.
Besides the above, theres also the issues of total population, Aliance:Hoard ratio, econony, stability, and longevity. Some servers have been around since the release of WoW, others are newer, coming out with BC, and I'm sure more will pop up at the launch of WotLK. There are websites out there which will give you most if not all of this server-specific information, if you just do a Google search.
Anyways, hope this helped. | 
