That's not left field. This is left field!

http://www.youtube.com/watch?v=OkaTiBHPiw8

You are not prepared!

Divinity II
Drakensang Series
Dungeon Siege Series
Jade Empire

I'm surprised no-one mentioned Nox yet as it been topical of late - RPG lite but not quite as much as Diablo!
Also the Elder Scrolls had a spin off called 'BattleSpire' which wasn't too bad - it was a level after level dungeon hack in the style of Stone Keep (Ooh - Stone Keep - there's another one!) rather than a huge world to explore which, I think, is why it didn't do so well
there was also an Elder Scrolls platformer called RedGuard, it wasn't great and wasn't really an RPG, I only mention it for the curiosity value...

Apart from some games that have already been suggested, such as the Ultima series (I would suggest you to start with either Ultima VI or VII, VI would be the best choice for starting the furthest away in time without getting too much of an outdated game -controls etc-, while U7 is definitely the gem in the series. I always loved 6 and 7 the same tho), and Arx Fatalis (really nice, lots of fun!), since you seem to be a nice fan of the Might and Magic series you might want to try M&M 8. It's not as good as 6 and 7 for a number of reasons, but it does have a certain flavour: you get to play in the non-human civilizations of the game world, with non-human or anyway non standard classes and races (vampires, trolls etc).
If you haven't played System Shock 2 however that should really be your first target.
On a more recent note, dont know if it's already been mentioned, you might also want to try Divinity 2 Ego Draconis (i think there's a pack with expansions floating around nowadays), it's honestly not the best game especially due to a bad combat system, but I love the way it doesn't take itself too seriously and it has it moments.