There are two major stages in character progression in a game like WoW: the leveling game and the end game. For most raiders (I dare say all of them), the end game is the place to be, the sole purpose of the game. Getting there is simply an annoyance and the sooner it is over with, the better. It's perfectly understandable. For me, though, the leveling experience is the game.
Ever since patch 4.01, I've been giving leveling some thought. What was the first thing I did when the patch hit? I leveled my new mage so that I could experience the thrill and excitement of discovering the new skills and talents. Since then, I haven't been able to stop leveling him. I'm almost completely neglecting my level 80 characters in favor of this new guy and he's past 50 now. Imagine, 2 weeks ago he was level 8, now he's over 40 levels "older".
I've always enjoyed leveling a new character, but paradoxically I want the levels to go by faster, so I can gain access to more powerful spells and talents. And then, one day my character hits max level and I feel a bit sad. As a solo player, the end game is pretty much THE END game for me. Yes, I have been running a lot of heroics thanks to the dungeon finder tool but if one thinks about it, that's just grinding. Grinding heroics so I can get better gear so I can grind more heroics so I can get better gear that will become obsolete anyway as soon as I step in the first zone of the next expansion.
Leveling on the other hand is exciting because there's always something new to be discovered. It's a journey of self-discovery, if by "self" I mean my avatar. I like gaining those talents, learning new spells and becoming more powerful. As my character grows, I become better at playing that particular class and every new level makes me feel more "leet" than previously.
Not all leveling is the same. I think that a lot of it has to do with the class and what I, personally, prefer as a class mechanic. For example, I really enjoy leveling my mage right now. I like the sense of power that comes from such a pure DPS class. I know for certain that I would enjoy leveling yet another paladin, as retribution. Who knows, in Cataclysm I might attempt it. On the other hand, leveling my rogue was a PITA.
Once my characters reach 80, things slow down almost to a standstill. I park them in a major town like Dalaran and I use them mostly for their professions and, depending on how fun that class is, for heroics. Well, dailies too. But from here on it's only grinding and more grinding. A very good example is my warlock whom I really enjoyed leveling as affliction. Once he hit 80, I only logged on him to do the jewelcrafting dailies. He's the most under-used level 80 I have.
These days, of course, I do most of my leveling through the dungeon finder. In a way it's a shame because I miss out on regular quest leveling and running through the different zones but I've seen these zones so many times while leveling other characters that I'm sick of them to the gills. Though I'm a solo player, I still enjoy the no-hassle approach to running a dungeon thanks to the dungeon finder. Now, running a dungeon is a very important part of my leveling experience, the one that gives me the most joy. The truth is that some dungeons do tend to get stale after running them a dozen times or so but so do regular quests. And questing doesn't reward you with the awesome blue items that drop in dungeons.
Now that Cataclysm is practically here, I find that I'm not as excited to level my level 80s to level 85 as I am to level some new characters from scratch. Perversely, I like it when levels fly by quickly because I can see my character grow before my eyes. Leveling from 80 to 85 won't be the same thing. Classes already possess most of their signature abilities and the only advantage of being max level at this point is the fact that you can run heroics and raids.
I'm very tempted to re-roll some of my existing classes in Cataclysm, just for the thrill of leveling them with the new talent trees. I'm particularly interested in a paladin (probably holy/retribution). Since Tauren will be able to be paladins, I'll probably start one of those. Unfortunately (as far as I know) we still can't transfer heirlooms across realms, which sucks. These days it's a pain to level a character for the umpteenth time without heirlooms. Don't get me wrong, I love leveling but only insofar as I can see progress, i.e. new levels coming in quickly. I don't much enjoy languishing at the same level for hours on end.
One thing that I've been thinking about is whether WoW will still be fun for me after I've finished leveling all the possible classes. Maybe not. As it is, I can feel that the day I quit WoW for good is on the horizon.