Good acting allows the audience to break through their disbelief. It is called a "willing suspension of disbelief" when you as an audience member become entrenched in the plot and activity of a movie that you have a viseral, emotional response to the show/movie. It is a bit of snobbery but most really good actors are those who are also capable of doing live theatre, not just movies.
In general, it is harder to do comedy because it involves timing and when forced ruins the whole suspension for the audience. Doesn't mean that comediec actors are better but that such performances are considered harder to give.
Breaking through and giving a good performance means allowing yourself to be vulnerable and freely express stuff that people think they keep hidden. It means being more aware of your actions and no real control over your words. On top of that, you have to deliver in such a way that people don't think you are doing that. I agree that good actors are well rounded and can play a variety of roles rather than just some derivation of themselves.
Sean Penn, Al Pacino, Morgan Freeman, Robert De Nero, are men I think are fine actors. I once saw a rendition of A Raisin In the Sun by Danny Glover that was fabulous.
Women are harder because a lot of times there aren't very many decent female roles for them to do--they end up being window dressing and the love interest. Cate Blanchet seems good. Helena Bohnamn Carter. Many of the older women of hollywood were fabulous but they have died off and no really new talented women have taken their place. I mean come on, Lindsey Lohan is playing Elizabeth Taylor?? Gross.
