Love it
I love this game. I love the strategy, the feeling of pulling off the perfect assassination, and even sometimes screwing up and getting your brains blown out of your head.
I've read some reviews that say you should be able to kill the guards, but that would just take all the fun out of the game. There are times, however, when it is almost impossible to not be seen by guards, and it might be a good ida to be able to dispose of some of them. Perhaps an item that gets used up when you kill a guard, that way it would be controlled. Some levels would give you zero of this item, and other levels would give you maybe two.
I've also read complaints about the randomness of the guards. If there was a pattern to all of them, again, the game would be too easy and not fun. But sometimes, when there are six or seven or more guards running around, you can be just sitting still for ten or fifteen minutes, which also isn't very fun. So I suggest the the green guy be on a random path, most of the guards be on random paths, but on levels where there are a shit load of guards, put some on a certain patrol. That way the game keeps its randomness factor, but not to an excruciating extent.
Anyway, these are just suggestions for a third game, which I really hope you make. Once again, I'm a huge fan of this game. It's awesome.