For me it's not so much the level cap than the fact we level up too fast - would it be possible to get the level 4 aroumd he underdark maybe?
If you do the critical path (i.e. Grove battle, find Haslin or his journal, then trip to the Underdark), you can even abuse your tadpole powers to avoid fights, you'll still be near level 4 by the time you hit the Underdark..
You get way more XP for advancing "remove the tadpole" quest (the main quest of Act 1) than anything else. Probably like that so people don't have to do all the side contents.
