What I like about levelling is the sense of having achieved something. I played in one 2E AD&D campaign for years, where XP and treasure were few and far between, and it seemed like there was never any progress. With d20, levelling is faster and there are more benefits attached. Better saves, better BAB, hit points, skill ranks. Having these quantifiable improvements on a fairly regular basis keeps me more focused and more excited.
I think that in 2E the benefits from levelling does not reflect the amount of effort required. My 7th level Ranger needs something like 75,000 XP to gain a level, and will then have a slightly better THAC0 and a few more hit points. My group games more or less once a month, so to level up within one year, I'd need to get more than 6,000 XP every session, which means that basically we'd have to kill a dragon every time.
And when a higher level means facing opponents with more magical attacks and defences, the minor improvements from gaining a level in 2E simply aren't enough. D20 assumes that your equipment also improves. True, that's very much down to the DM, but I like that it's been incorporated into the system.
So essentially, I like it when your character visibly improves with a level, to be able to meet new challenges, and when you can look forward to reaching a new level within a relatively short space of time and without requiring ridiculous amounts of impossible victories.