Did it felt boring doing the same stuff you did last time? Did you had more fun since you know your class a lot more? Each time you level a character, your knowledge of the game increases. So, even if you were to level one character to 80 and then immediately level another in an identical fashion—same faction, class, race, quest progression, etc—I imagine that your experiences would still differ greatly. If anything, the second time around, you'd have a better understanding of your class' mechanics and could melt face with awesome efficiency. Who could argue with that?
Speaking personally, though, I've leveled a number of classes more than once and I enjoy the process immensely. There's so many different ways to play a single class, and it can be a fun challenge to discover them all. It's a novel sort of familiarity—even if think you know everything there is to know about priests, or shaman, or rogues, since you're playing at different experience level and in different environments, there's always the opportunity to learn something new.
Like Kaneka noted, so long as play what you enjoy, it's hard not to have a good time. In the end, though, you won't know how you like leveling a class more than once until give it a try. :)