Its perfectly fine. Why do you think there are so many Minecraft clones around?
Though I agree, its like "I might be stealing Wolfenstein 3D's ideas, my game will have weapons and it will be played in first person!"
I'm against clones in the sense of those games that only strive to get to the sample place exactly the same way as another game. That's a rip off. But honestly, who never thought "Hey, I like this game and I know how to make it better!" ?. I'd say that most game ideas (or ideas in general) begin with that instead of coming up with a game idea out of the void.