The Quijote, which is acknowledged now as the Greastest Modern Novel written in Spanish, was dismissed by the writers at the time as “non-art”.
That's a good point. I don't agree that we can't identify what's art until later generations decide, but I do agree that time plays a big part. A lot of the greatest works of art didn't start out as great art (take Casablanca for instance). And the converse is true too: one of the surest ways to end up with crap is to start out convinced you're creating one of the greatest works of art ever.

I’m irritated by the fact that Ebert is focusing on assorted definitions of art and games, which are themselves not truly nailed down nor universally shared, then attempting to classify specific examples as one or the other based on their attributes, as if the definitions were set and agreed before the debate started. He might be worth listening to if he was commenting on Flower or Braid or really any other game at all after having *played* them.
Well yeah, which is a big part of why so many people are jumping on the chance to correct him and say stuff like "just play Passage or Flower or Planescape Torment and you'll get it!" even though it's pretty clear that he wouldn't, even if he were interested. Any more than somebody could tell me to watch "Glee" and suddenly i'll appreciate the appeal of the show.

The worst is his bit with Flower. He tries to say games aren't art and then criticizes that one for not being a game. It is both a game and a work of art, but he's changing definitions to suit his own argument. That's why arguing with Ebert for the purpose of convincing Ebert is pointless.

Don’t get me wrong; I think it’s important that video games be classified as Art so they can be protected under the same laws as porn. But otherwise I think the argument is a waste of time. Will better games be made if people approach them as “art” rather than “games”? I doubt it. There are many smart, talented people working in the video game industry, and it is an entertainment industry, just like the film and art industry. You don’t get into the entertainment industry if you’re not motivated, if you don’t have any ideas. So you have a group of smart, talented, motivated people full of ideas who are working their butts off; do you really think they aren’t already trying to make the best games they can?
There are many more people working in industrial design who are even smarter and more talented. Take the automotive industry. They're working their butts off to make the best cars they can, and they can do amazing work. But their goals aren't the same as the entertainment industry. Making the best product possible according to the needs of the customer is great for most industries, but it's deadly for art.

And there's so much pressure in the games industry to do exactly that. Even more than the most business-driven film studio, since the nature of games is still so nebulous, and because games are so much more closely tied to the audience. It's already common to hear "audience" turned into "customer" or "user," and "game" turned into "product." In that kind of environment, it's not just that there's little incentive to use the game as a medium of expression; there's active resistance.

After all, isn't that the more noble way to look at it? The customer is always right. Games are about enabling the player. Anything else is purely ego-driven on the part of the developer. It's not about you; it's about making a good game. And besides, it's just a game -- it's not anything important.

And I think that's exactly why it's important to keep asserting that games are art. Not to convince Ebert, but to remind the players and the developers themselves. It's so easy to just dismiss playing games as idle entertainment, a waste of time (even when compared to "real" culture). Or to become convinced that expecting games to be an outlet for self expression is hubris or ego, or getting all uppity and pretentious by trying to put deeper meaning into a game. You can be extremely motivated and talented and make fantastic stuff that still isnt art, if it never occurred to you that what you're doing IS art.