I've found this article quite fun to read and I suggest to everyone to take a look.
It describe how working on Spelunky (dont' ever play it? Go and play it! It's free, open source and fun!) help developers to think about gameplay working on small prototype, have feedback from peoples and go on, in more and more iteration finding a good balance and perfection.
It's important one think in the article: how passion in one genre (rougelike) have been mixed into another genre (platformer), creating something unique: really, really inspiring!!
I wonder what kind of "mix" of different genres can be done, for example: mixing together puzzle games with first-person-shooter (Portal anyone?) or mixing tower defense with rts?