I only consider Fallout and Arcanum to be true RPGs, the former being among the greatest games I played and the latter among the mediocre ones (but not bad nor worst, though).
A well-translated P&P RPG experience is that, what a P&P RPG experience should be, but on computer. There's a DM and a campaign (the game's script), a ruleset (designed/adopted by the developers), and a player (who controls one or vaarious characters). During said campaign the player can do as he wishes: he can slaughter whomever he meets (if he can), choose to deviate from the main quest and follow some menial side stuff for a while, explore the world openly and get to know different locations, design a character with a lot of freedom during the process, flesh out his character's personality throughout the game, and affect the world he/she interacts with.
You can't do those in Deus Ex. Fallout and Arcanum, on the other hand, did all of that commendably well, except Arcanum's quality was abysmal in the combat, ruleset, story (not to be confused with atmosphere and setting), and dungeon design departments.
PS/edit: And yes, I consider a game like Oblivion more of an RPG than Deus Ex. You can do most of the stuff I mentioned above though it's as if the DM were incredibly retarded with a campaign & ruleset of the worst quality. C&C is on par (flavory flavor) and so is the story's linearity. The story itself, however, and the atmosphere, writing, and "dungeoneering" are so much in favor of Deus Ex it's not even funny.