A lot of players who hit 70 (especially in a lot of serious guilds) only have one character.
I nly played one character for ages, and then got bored recently waiting for WOTLK, and people said they were bored too so helping me boost constantly :/ got bored of that too though so far, so I am just twiddling my thumbs waiting now
I have:
70 Warrior
70Lock
70 Rogue
70 Palla
My warrior is my main - MT for guild so i'll partly get boosted , i'll be leveling him first, then my rogue as hes my gold/pvp machine - should be a nice quick grind on him. Then i'll prob level my palla, so i can kit up as a spare healer for my guild.
People in serious guilds usually have more than 1 char, as they won't play on there main 24/7. Personally on my main, i don't do heroics, or anything like that anymore, all i do is raid. My alts are where the fun is in.