The app isn't live as we write this (it arrives later on the 16th), so it's hard to say how well it'll work in practice. However, it's clear that the folks in Mountain View believe that Google+ communities can only accomplish so much. They tend to revolve around larger groups, and it's easy for someone's thoughts to get lost in a sea of posts. Spaces is the antithesis of that: it's for book clubs, developer meet-ups and any other close-knit community where every person's input is valuable.