This is all very reminiscent of the video game industry of the 80's, where everyone was making largely the same games and trying to figure out where they fit in the marketplace. Some lawsuits like this were successful, and others weren't. It's a tough call, though I tend to side with Zynga here.

Copyright on characters, art, text, code, and logos is great. We obviously don't want direct clones. But, Copyright applied to a genre and style of gameplay is ridiculous. We wouldn't have any of the great games we have today if those things were enforceable.