I could say he wasn't shown the best examples of artistry in game design. While he only saw Braid for its mistake-reversing element that seems like cheating, and presumably didn't realize that most of the game is about using various time-manipulation powers to solve elegant temporal puzzles, I would agree with him that the game's still little more than an arcade puzzler with pretentious write-ups between every level.
I could say all of that (and indeed just did) but none of it matters. You know why? Because art is subjective. There has never been a clear definition of what exactly "art" is, and that's because it varies from person to person. There will never be a consensus on "videogames as art." I doubt there will ever be one over the matter of Tracey Emin's Turner-prize-winning dirty bed, either.
My personal definition of art is something that provokes emotional attachment. And there are games that have given me far stronger emotional feelings than any other story told in any medium. Fear, despair, joy, sympathy, the whole gamut. But these were all extremely personal experiences. I'd no doubt have felt differently if I'd had a different personality. I can't really share the emotions of a film critic blubbing at the end of It's A Wonderful Life, and I don't expect them to share those of my eight-year-old self blubbing equally hard at a funeral scene in Wing Commander. There are no doubt people reading this who were moved to tears by Aeris dying in Final Fantasy 7. I can't sympathize any more than I can with Roger Ebert, but I can't tell you that you didn't have those emotions, or that they're somehow wrong. And Ebert isn't "wrong," nor is he "right." His perspective is just that - his own.
And that why it's stupid to get angry and butthurt about it, or anyone else who dismisses gaming. It speaks more to your own insecurity than their obvious ignorance. It puts me in mind of evangelism. A religious person will (generally) never, ever convince an atheist of the existence of their god, nor vice versa. Religion should be something you keep within the confines of your own head, and we should all recognize how pointless it is to try and make other people see the fairies that live in your brain. That's how I feel about art.
TL/DR: Art is any created work that provokes strong emotions in you, personally. And trying to impose your feelings on someone else is as pointless and time-consuming as trying to impregnate a dishwasher.
Yahtzee is a British-born, currently Australian-based writer and gamer with a sweet hat and a chip on his shoulder. When he isn't talking very fast into a headset mic he also designs freeware adventure games and writes the back page column for PC Gamer, who are too important to mention us. His personal site is www.fullyramblomatic.com.