not sure if its on pc but kingdoms of amalur is brilliant, its got all the bright graphics and characters like wow but it doesnt involve other players unless you want it to, or maybe the fable games there really good
The Orange Box aka Half-Life 2 compilation - by the time you've got through HL2, Episode 1 and Episode 2 it'll be time for HL3 to be released (plus it includes Portal and Team Fortress 2)
difficult once you have been in World of Warcraft to find one that satisfies the gameplay and interaction with the "world" that WOW gives. Cant help you as never found that elusive alternative!