I would like to point out the One Video Game Roger Ebert Loved.
I think that he had a preconceived notion of what games are. Some games are not art, clearly. Someone mentioned the rules and mechanics of a game limiting the artistic expression of a game. That there are imposed limitations on what can be done that remove the artistic nature of a game.
Someone mentioned that the concept could be applied to movies, but I think that dance or music can be a better comparison. With music, you can honestly blurt out any sound you like, but there are rules as to what can be done to make a song sound nice. We impose limitations such as keys and time signatures to add the structure necessary to make it Art.
I feel like gaming has changed dramatically since Ebert put out his call against gaming as art and we have seen numerous titles evoke emotion, leave room for expression and allow players to experience things in their own way. We leave things up to interpretation constantly. A man can look at a work of art and say, "This is not art," and another could say, "This is art."
For Ebert, games can't be art. That is fine. For me, they can and I am happy with that. It is what it is.
Also, if you want an example of Game Rules as art, play Graffiti Kingdom where the rules are, "You draw monsters and based on what brushes, color and shape you make it determines their stats and abilities." I made a strange, thin dark looking beast with a tail and thin runes etched along its body. I also made a penis with scrotal legs and feet and it shot out yellow lightning blasts and lightning sprays. Hilarious.