John Candy's comedy had a very gentle edge, and he was quite a good actor. Movies like "Only the Lonely," and "Planes, Trains, and Automobiles" showed what a talented actor he was, who could really touch your emotions.
Mark
That's not to say that Belushi, Farley, and some others didn't have a kind of genius; it's just that they needed to restrict themselves to roles that played to their character. It's kind of like Curly from the 3 Stooges. I couldn't imagine him ever playing a dramatic role, but there was never anyone else that could have pulled off some of his scenes. I don't think anyone else could have pulled off that scene of Bluto looking in the sorority house window with that little double eyebrow raise and made it as funny.
On a side note, Jane Fonda gets a lot of kudos for acting ability, but I've never seen a movie where you believed she actually cared for anyone. Even in "Barefoot in the Park", you never believed she cared about her husband.