As the title says, what gave you the most motivation to level the first time you leveled?
For me, it was the monsters you would now and then meet in low level areas that were elite and often more than 10 levels higher. The knowledge that one day, I would kill them for killing me when I was a low level, a feeling most new players cannot experience, I imagine, as the leveling is far more streamlined (the story would probably feel way more compelling though).
The second time, I think it was the vision of how easily I will be able to own noobs in BGs (as shaman in vanilla, doh), but that's a little offtopic.
What was yours?