If I have a boost I use it, if not I slowly level. I don't change mains that often. Think 4 times in 10 years and probably half that time I was Main/Alt between only 3 characters. Just added one this expansion. So I'm never in a hurry to max level a character. The only main I've ever retired totally was my priest, I just end up in guilds with a ton of priests, so it was always better to be a lock, DK, or now, warrior.
I think I'd like to have a Shaman healer at some point, just cause. or maybe a monk tank/dps, again just for fun.