Here are the only zones and side-quests I did:
Blighted Ruins (the undead starting area)
Derth Arena quest (easy bonus)
Sandworm Lair (great bonus)
the ruined dungeon (in search of helpful loot)
Dreadfell
Reknor + Unremarkable cave
the Orc Prides
Eruan + Charred Scar
Slime tunnel + High Peak
As a completionist at heart, I still cleared every vault and killed every enemy I found. This was my mistake. If one wants to win at below clvl 50, then don't open every vault and don't kill every enemy you find.
A few under-leveled highlights:
Defeated the Master at level 14
Went through the Charred Scar at level 26
Defeated all the Orc Prides at level 33
Entered the final battle at level 40
Closed all the portals at level 47 (Aeryn and Elendar had already been killed)
Also killed the final boss, Argoniel, thereby winning the game, at level 47
So, is a sense, I did beat the game below level 50, but I was level 50 after I beat the game

(One final note: the invisibility talent is way too good right now. At the very least, increase the mana drain by 1)