You don't have to do any quests to level up. I think it's safe to say that if you actually explore the dungeons, rather than stepping in and leaving quickly, you will start leveling up very quickly. Especially because you level up about twice as fast as in Oblivion.
You're right that it probably won't be a big issue....but I still see the possibiltiy of "breaking" the game by visiting all the dungeons just to have a look at them, and the enemies, and then leaving to find another. If it is true that there are no high level preset dungeons, someone could do just that.
I certainly hope it is a mix of the level-locking dungeons and a bunch of preset higher level ones. Because if even 2/3 of them are level-locking, I think many people will be upset at the lack of enemy variety, and it will be silly that the world basically conforms to you. To me, that's the same problem with the bs OB had with the entire world getting stronger and different because you leveled up. Both would be the same issue, one would just be the lesser of two evils.
They can avoid that by having roughly half of the 130 dungeons preset as special leveled dungeons that are locked to a certain level of enemies no matter when you visit them. That will leave half like OB's, except with level-locking, and the other half will be a mystery until you get there. I'm not sure they will be implementing anything like that half and half scheme, but I'm sure most people would be happier if they did. I know I would see that as a better system than MW's or OB's because it compromises with concepts from both games, making for a more varied experience.