Incredible game, except there's no skill involved.
Okay, this game had a lot going for it. Decent graphics & sound. An incredibly efficient (I'm guessing Bitmap-only) engine. Tons of stuff happening at once with no slowdown. Delightful emergant behaviors (try firing the laser after using the triple ship powerup for a poor-man's bomb!) and seemingly unlimited waves of gradually-increasing difficulty. Yet it never quite seems to become unbeatable.
So what's the problem? By level 50 or so the game has already accelerated to the point where seeing what's going on, thinking, planning, and reacting just isn't humanly possible. There's no gameplay, after that point, except holding down the rotate button, keeping three ships active at all times, and spamming the special attacks.
Soon after that, the various powerups are appearing and disappearing so rapidly that the only thing you can do is mash the 1-7 keys indefinitely.
I do like the fact that the game was able to keep going forever. I finally quit after level 700, and today's top score apparently played for 58 times longer than I did. Scary. Despite the fact that the game remains (technically) playable pretty much forever, it's only fun for about a minute or two, before rapidly accelerating past frantic and into button-mashing spamfest.
Usually when I see scores this high, I assume the highscore table's been hacked, but in this case it's pretty obvious that anyone can score that high if they sink enough time into it. If I had the right macros, I could beat the top score with duct tape. The only limit is Flash's max integer value.
All that said, I like how the game never beats you over the head with failure. Grainsalt came very close to an ideal game design, here. IMHO the only problem is that the gameplay is a bit TOO streamlined. I recommend breaking the gameplay up a little bit, next time. Do five waves, then display a shop menu where the player can buy powerups. Then the next five waves, maybe there's some different gameplay rule or foreign object in the arena that changes the gameplay a bit. Maybe every hundredth stage could be a boss, I dunno. My point is, the diversity available within the game was all packed into the first few minutes of gameplay. Ideally you'd want to spread that out over the course of the game.
If the game has infinite length, you need an infinite number of permutations, sprinkled liberally over the course of the game. I attempted this (clumsily) in one of my games, and I don't recommend it. Probably Diablo II came the closest out of any game I've played, but even then it didn't truly last forever.
I guess the moral is, spread your content a little thinner next time. Don't add a time limit or a surefire lose state, but do make it an ongoing campaign with breaks, a way to save your progress, and notably different gameplay over time.