Taken straight from the open Sunday thread, we had an interesting discussion about somebody with 4 level 70 characters, who was asking himself which character to level to 80 first in Wrath of the Lich King. Good question, I have 3 level 70s myself, and as even my ultra-casual wife has 2 level 70 and one level 68, I guess there are quite a lot of people with several 70s around. So which one of them is your "main", which one to level first?
Of course I can't pick a class and say "play that one". But I can give advice on how to choose. And my advice is about the fact the same character class might be fun in different degrees depending on what you do with it: Leveling up, grouping, raiding, PvP, or whatever. And I'd strongly urge you to consider what your main is going to do the most over the next 2 years, and take the class which will be most fun for your long-term main activity, not just for leveling.
That is why personally I will level my holy priest up first, in spite of the fact that since the patch 3.0.2 even my warrior is more fun to level up, and the mage would even be more fun. But leveling up is something I will only do for a relatively short time, several weeks, a few months at most. But with nobody seriously believing that Blizzard will bring out another expansion next year, I'll be stuck at 80 for a much longer time. And there I mainly want to group and to raid, and that will be more fun for me with a priest. I like healing, I like group play. The fact that holy priests still have one of the most boring ways to solo combat (pull - bubble - wand) shouldn't stop me from playing my priest first. The characters that are more fun to level, I can still level as alts later.