Perhaps a sin to mention games you haven't played yourself, but I'll risk anyway. Wizardry 8 seems to be one of those RPG's that's mentioned in a lot of favorites from RPG developers.
Other than that, yeah, I wouldn't add Diablo + clones, Dungeon Siege and Spore to an RPG list (especially not one of essentials). And Oblivion and Fallout 3 aren't part of any essential list I'd make. I'd even dare say NWN 1 & 2 aren't really that essential, especially from a story perspective, but Mask of the Betrayer should be a must, for sure.
Nor would I add The Longest Journey and Dreamfall to it, but kudos for mentioning them anyway, Avery.