The number of levels itself is not a particular issue for me... 10, 20, 100?
I don't like to level up too fast, my guideline has always been "if you level up before you even had a chance to try out what you got at the previous level, then probably it's already too fast". Levelling up too fast means you end up thinking more about what powerup to get next, and less about how to USE your current abilities.
OTOH I don't think I'd have problems playing in a system where there's lots of levelling, but each of them is really minor, or even a level-less system.
If you get a little every time, and it happens often, it's ok for me.
If you get a lot every time, but it happens only every now and then, it's ok for me.
I'm much more concerned by the TOTAL power gained at a fast rate. I am definitely not fond of characters that go from pathetic apprentice to world #1 archmage in six month of their real life, or in just 2 adventures.