By all appearances, Sims Carnival is just one more user-generated game portal, following suspiciously close to Microsoft's announcement of the XNA Community Games. What may set Carnival apart, however, is the involvement of Rod Humble, last seen wowing audiences with his minimalistic art game The Marriage.
Still, examples games shown include your standard Space Invaders clones, and other simple titles which could very well turn Carnival into another dumping ground for mediocrity (albeit with the occasional low-fi breakthrough). The program is currently in a closed beta, but users can sign up via the website. We're anxiously waiting to be proven wrong on this, so go forth and create.