The most difficult thing I found in game making (a part of the pitfalls in programming and so on) is to figure out if the game is actually playable or not.
Maybe it's just me, because I never was a real big game player, but I don't have the feeling for the fact if my game is too easy or too difficult. The first level in my actual project is just finished. When I play it, I win every second game. There is a lot of randomness in, so that sometimes, it's just too many ennemies to reach the required killing score. But I by now KNOW what they do and what not.
I will probably know more after people have tested, but how do YOU know, whlie making your game, how to set dfficulty?