Maybe discussing “What IS art?” isn’t interesting to you, but I still think it’s important to the discussion because you can’t determine whether or not games are art if you can’t define what art is.
You need a definition, but a personal definition, not an objective one. Every time this topic comes up, it gets derailed by people saying "define 'art'," and it never ends well: you either come up with something so vague it includes everything, or that includes only the stuff you like. There's simply not going to be consensus, and that's why I say the question isn't interesting.

Instead, you come up with your own definition. Santiago borrowed one, Ebert borrowed a different one, and I presented the makings of my own. It's a lot more useful to take the definition that's important to you and then point out how something does or doesn't fit.

I will say that I think the "layman's definition" you present is way off-base, because it is -- as you point out -- based completely on validation from someone else. And -- as you don't point out -- the assumption that other people think this way is condescending at best.

The points you bring up, that a “great game” doesn’t have to be art, that the developer’s voice isn’t important, that’s not arguing that games aren’t art, that’s arguing that they shouldn’t be. Convincing those people that games are art won’t change their approach to creating games.
Well to be clear in case anyone hasn't been reading this blog for long, I definitely don't agree with the opinion that the developer's voice isn't important, or that games shouldn't be art. But I do believe that somebody could approach games with that philosophy and still make fine games. I wouldn't agree with their philosophy, but you couldn't say it's "wrong."

And I disagree with your last claim: I think the mindset is extremely important to how you go about making games. My first job in games was at a studio owned by a film company, and there was a pervasive mindset in the company that a hierarchy exists: film on top, games on bottom. You'd get the sense that some people got into games because they couldn't quite make it in movies. Or that artists would separate their "real" art from the work they did for the games.

If you don't see games as an artistic medium, then that doesn't mean you won't make art, just that you won't use a game to do it. It's like the guy making comic books at work and saving his real creativity for the novel he's writing on his own, or the person working on a sitcom to pay the bills while she saves up enough to make her indie film. They're not "phoning it in" if they don't believe that comic books or sitcoms are the proper medium for deeper ideas in the first place. That's why I think it's important to keep the idea alive that the game itself — not just the music, or the background paintings, or the concept art — can be an outlet for artistic expression.

That’s what I think is the biggest difference between games and other medium of expression. It’s not that games are interactive and other medium aren’t, because all art is ultimately an artist’s attempt to interact with their audience. It’s that games are far more interactive and gives more responsibility of experience to audience than any other medium. I don’t just think that games are art, I think they have the potential to be the most engaging form of art yet known.
And that's basically why I think that the question "Are games art?" is so much less important than the question "Why do I care if someone says they aren't?" Because if you just take the stand "of course they're art; that's trivially true," then you haven't really gotten anywhere. Or if you say, "They're art because they have so much art in them," you haven't said anything unique to games.

But when you ask "why do I care?" then you start to get at the more interesting questions: What is it in games that I'm looking for but not finding? or What is it about games that makes me convinced that they are, and I'd like to see more of it?

And yeah, I agree that that interactivity is the key thing that's unique to the medium and that needs to be exploited. (And if that seems trivially true, the consider the fact that a lot of really good games still take the form of movies with interactive bits tossed in to break things up). It's not rules, or objectives, or a non-predetermined outcome, or even (I would say) choice, as much as the idea of a work of art that can make me feel something based on what I did, not just what I watched or read.