Aviary's filters are actually quite bad (I've done a bunch of research on this because we open sourced a live filter camera picker for iOS) so I'm not sure what Twitter is trying to accomplish with this either.

That being said, I think good filters are quite powerful. Instagram's filter offering is much better than most free filter apps. While not the secret sauce anymore, originally I do believe it was a huge part of it. If you look at Facebook Camera and compare the quality of the filters, maybe you'll see what I mean.