Ebert is a fantastic, very thoughtful critic. Him and Kermode are the only two critics I read / listen to who seem to convey a sense of enjoyment in doing what they do for a living.
Ebert doesn't make critic, don't have a criterial or a normal way of evalute movies outside stars or numbers at the end of his essays. You can have all the knowledge you want but if you are unable of making arguments outside "good acting", "i like this", "i don't like that" or "this is boring" you aren't a critic and you're just giving your personal opinion and toughts, wich is fine but it's nothing that you or me can't do and no one should be getting paid for that or being named a professional "critic".
Critics should be about analysis and forms of convay toughts and seriouss discuss about the object you're writing is or seems to be about. There are plenty of ways to do this, analysis with psychoanalysis, Structuralism, semiotic, etc. People like Rosenbaum or Martin are respectful critics, people who seems really concern about create new paradigms about cinema wich is valuable even if don't agree always with them
Kermode is even worse, i will never judge a movie for vestuary, what the fuck is that.