You know, I can't speak for anyone else, but I am always more impressed by actors who make you forget that they're acting. Kevin Spacey did that in "American Beauty." Tom Hanks did that in Philadelphia and Saving Private Ryan.
And Paul Giamatti does that in Sideways. It's a marvelous acting job, and shows a delicacy of touch. It would have been very easy to make Miles merely pathetic, and Giamatti does not do that. Miles is very real, very human.
Contrast that with Leonardo di Caprio in The Aviator. Yes, it's a good acting job, but not a great one. His Howard Hughes is all neurotic twitches -- you don't get a sense of him as really human. (And yes, I know di Caprio was playing the most famous neurotic recluse of the twentieth century, but it still has a showy "I'm acting" feel. Especially later in the movie.)
Guess which one the Academy of Motion Picture Arts and Sciences chose to honor with a "Best Actor" nomination? Idiots.
Of course, at some level it's probably academic, since I'll give you any odds you like that they give the award to Jamie Fox for channeling Ray Charles.