And to be topical... Robert, you may be interested in a brand new proposal that Alec put forward to the TIGSource community for a little thing called the Monocle Engine. It's super exciting and speaks to some of the questions you've put forward.
Hey, glad to see that I was able to spark something of interest over there. As I said, I'm not a C++ developer myself, so it's not the engine itself that I'm interested in personally for my own development (though that would be awesome). I just want it to
exist because I know from a developer standpoint that one of the biggest obstacles to writing a game is working out the mundane stuff like input, audio and graphics rendering. That's a lot of time writing
plumbing for your game instead of writing
your game, and I imagine it's something that ultimately kills a lot of indie projects. ("Hey, Alec already knows all this." "I know. I'm not telling
you, I'm telling
our viewers!") If such an engine exists, more games get made, which is good for those of us who like to play them.

One thing I didn't see mentioned in the engine description on the TIGSource thread: networking. Aquaria had no use for it, of course, but a lot of indie games would.