From a very early stage I had included an overheating mechanic in QANAT. My thinking was that I wanted to dissuade the player from just continuously firing a wall a bullets up at the aliens. As you fired the gun it heated up and if you allowed it to reach a critical temperature then it would shut down for a few seconds.
In my own testing I found that this didn't have the exact effect I was looking for. It didn't really make me pay attention to keeping the gun cool but it did result in the gun overheating every so often. When the gun overheated I would have to switch to trying to avoid the aliens' bombs until I was allowed to shoot again. I rationalized that this was good - the fact that I hadn't managed the gun temperature meant that I would have a nail-biting time trying to stay alive with no defence.
After the modifications in v0.4, where the alien ships would crash to earth when you shot them, I finally realized that this mechanic was a really bad idea. The driving mechanic of the game is shooting and taking away the power to shoot takes away the core mechanic. The four seconds of avoiding stuff is not really fun, it is just dead time while the gun cools.
In v0.5 I tried a new approach to overheating. I wanted something that felt somewhat realistic while at the same time added to the core mechanics. The new behaviour is that the overheated gun continues to shoot but the bullets move more slowly and some of them will fracture into three bullets moving in a random direction. When you first see the mechanic in action it will appear as though it makes the overheated condition actually advantageous. The fracturing bullets can hit aliens over a wide area. However, the subtlety is that this actually creates more chaos, and therefore makes this a more dangerous condition. You will often die during an overheat because you bring down aliens in a wide spread and it is hard to avoid their falling ships.
The original approach was a direct punishment, removed a core mechanic, and wasn't fun. The new scheme extends the existing mechanics of shooting and avoiding falling debris. When the gun overheats the game changes slightly, becoming more chaotic and more challenging and more fun. I think it works well but look forward to seeing feedback from others whether this is successful or not.