Take a rail shooter and subtract everything but the bosses. That is the basis for “Warning Forever.” When I first heard that as a discription of the game, I thought it sounded different, but unexciting. Then I found out the bit of genius that makes this game so worthwhile. The bosses adapt to your style. That is to say, the way you beat one boss will play a role in determining the next one. Kill it one way and its successor will have more lasers. Kill it a different way and it will have wider wingspan. And so on. This increases the replay value tremendously.
You have a time limit. Your ship is vulnerable at a one pixel area. If you are hit in that area, you die, are respawned, and lose 20 seconds. Each boss you beat will restore some time. You keep going until time runs out. That’s it. Simple, elegant, fun. The game is small, and free to download.