With Google+ Communities, Google is essentially just building forums into/on top of Google+. It could work beautifully, replacing the act of going to Chowhound and a bunch of other separate, super-specialized kinds of forums with one product that simply spans a bunch of niches — except that I wonder what it'll mean for Google Groups, Google's current forum/discussion board product.
A Google spokesperson offered only that Groups is "going strong," but historically Google+ moving into the neighborhood, product-wise, has resulted in colonization of existing products: Google Talk video chat got merged into Google+'s Hangouts; the existing Google Local was replaced by Google+ Local; and Google Reader was effectively gutted. Google+'s gravitational pull on Google products is strong because its ethos is "about building one seamless experience," says Google Senior Vice President of Engineering Vic Gundotra. In other words, it lives to consume every part of Google that isn't already a part of it. So it's hard to see how Groups will escape the gaping maw of Google+ forever, one way or another. But damned if Google+ Communities isn't a pretty experience.