There are games in which kills > 1 receive decreasing levels of experience. There are also games (notably FFXI) wherein the same creatures give less experience (up to eventually 0 experience) upon leveling up. FFXI's system kind of sucked from a practical viewpoint (early ffxi was a GRIND), but I always liked its "realism."

What I would like to see is a game with separate systems for each of a character's parameters. As in: strength is leveled up through repetition, intelligence through varying inputs (in other words, systems of diminishing returns for repeated patterns of actions, greater experience for variations in actions) [reading the same book over and over doesn't increase intelligence as quickly or efficiently as reading different books each time], charisma by talking to different people in different situations (in TES, as a Breton, talking to a Nord would not be as much a test of charisma (personality) as talking to an Orsimer would be, and would therefore provide less "experience"), etc.

Edit: yeah what Mirage said. Speaking of which, do FFXIV's systems approximate FFXI's?