You've probably heard that it's a good idea to playtest your game. If you haven't heard, well, I'm telling you now. It's a good idea.
But how about playtesting other people's games?
I was reading a book called Don't Make Me Think, a classic in web design and usability. At one point, the author suggested doing a usability test of a competitor's website before you start designing your own.
Why not apply it to game development?
If you're thinking of making a particular type of game, find an existing game that may have some relevant similarities, and do a playtest of it.
Find a random person, sit them down in front of the game, and observe. Watch for what they enjoy, what confuses them and where they get frustrated.
Then repeat with other random people and other games. Do it with both good games and poorly designed games. You will learn more by testing both than by only looking at one or the other. See where the bad games fail and what the good games do differently.
And then when you go to design your own game, you will know what mistakes to avoid.