I was reading The Grumpy Elf's last post and it made me think.
When MoP comes, I'll probably level my mage first. She was my second 70, my first 80 and my first 85. It goes without saying by now.
But by that time, I'll (hopefully) have five or six 85's waiting to reach the level cap, and that's only on my "main" realm. I probably will level all my characters to 90 at some point. Because I love levelling.
Here is how I plan to level my characters. Most likely in this order.
Mage : Mostly through quests and dungeons. I'll probably run each dungeons at least once, for the quests. I'll also do as much Archaeology as I can. Because I hear it's good xp. And I can always go back and do the quests after I've reached the level cap. Moar munies. This character will be leveled up with one of Mad's characters. So I'll have to wait for him.
Druid : Gathering plants until I get sick of it and then some more. Depending on which day the launch is, I might start this right away. If Mad is working, I'll spend every moment I can picking flowers.
Hunter : Mostly through quests and dungeons.
Shaman : Questing. Maybe some dungeons. I want to skin ALL THE THINGS. So I guess questing is better than running dungeons.
Death Knight : Questing. Occasional dungeon.
As for my paladin, I'm not sure yet what to do with her. There's a guild on my server that is for 70's only. I might turn off her xp for a while. Whenever I feel like doing some BC stuff, I will have a toon ready! I've never really did endgame content in BC. This could be fun.
And the rest of my toons will be levelled after that, when I get bored with the ones I have, or just because I feel like levelling something. Of course, a panda monk will be rolled somewhere in there. At some point.
And following The Grumpy Elf's advice, the moment the expansion goes live, I will log on all my characters so they can start building up rested xp. It won't hurt. I don't know yet if I'll level multiple characters or just one at a time. It's a good idea to just switch whenever your run out of rested xp, but I want to have at least one 90 before switching to another toon.
Now let's wait and see in a couple months if I'll follow this or not.