You may or may not be aware of the fact that you are only allowed ten characters per realm. For some, this is not a problem, and for others it is. Kinless writes about his dilemma, with the upcoming expansion, in which he will find himself wanting 13 character slots on a single realm. (I feel his pain -- I'm going to have to pick a character to delete in order to roll my expansion Blood Elf on my usual realm. And then I'll have to delete a second character if I decide I want to try my hand at playing a Draenei!)
Of course, Blizzard allows for this, in a way. You can always put your alts on a different realm, which has the notable side-effect of giving you something to do when your usual realm is down -- but that means playing away from friends and guild-mates. Starting over on a new realm isn't for the faint of heart: when you've made a name for yourself on your old realm, and regardless of your alt's name you're likely able to find friends and help when needed.
So is ten character slots per realm really enough -- or do you find yourself wanting more?