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Unlike other popular MMOs where you level up to 60, 80 or 100, D&D only allows you 20 levels.
Only 20 levels? Well that must severely limit gameplay!
But it doesn't. Players get new abilities ("Feats") at little sub-levels along the way as you progress from one level to the next. So while you're not technically "leveling up" and getting more HP & Mana, you're getting some benefits as you play.
One huge, unexpected boon for having only 20 levels is the massive number of players with whom you can group for adventures.
Going up a level takes many, many hours. In the time it took me to get to level 3 in one of the faster leveling classes (Cleric), I probably would have been 15-20, if not higher, in another MMO. So while in that other MMO I would have a small number of players to group with, in DDO I can realistically run dungeons with players level 2-4. And there are massive amounts of players at those levels who are interested in running the same content as I am. It's the equivalent of being able to group with players from level 5 to 20 in another MMO, and the content we're running is desirable for all of us.
This is a pretty great feature I hadn't expected. I thought only 20 levels meant a limited experience. The reality, however, is quite different. It means a much larger pool of people to play with as you work your way through dungeons.