Other than the Oblivion style level scaling that we already know is out, I don't really know enough to have a firm opinion on what will work and what won't, so I'll put my trust in Larian to handle it. They've got experience with it.
The only thing that worries me slightly is the experience splitting between party members, summons and companions. Larian has said clearly that only those who participate in a fight get XP (I don't know if a character who dies in battle gets XP for enemies killed after they are.)
My worry about XP splitting is that if you make too much use of multiple henchmen/companions and keep swapping them out, they'll all end up under-levelled. That's only really an issue because I don't know how much XP is available in the world. Is there a generous enough amount to get a party of 4 to hit the XP cap well before the end-game, or will things be tight enough that a party of 4 - even the same party of 4 used all game will just barely hit the level cap in the Definitely Very Final Dungeon?
I remember the 2004 Bard's Tale with its Level cap of 21. It had respawning enemies in places, random encounters on the world map and some re-enterable dungeons, and even then if you did basically everything, but didn't do a lot of world map random encounters, you could fall short of hitting the level cap.
