GDC way back at the beginning of this year, an MMO was all the rage that a lot of hardcore players probably haven't even heard of. The game was called Sherwood, and it was a hugely successful 3D game made only by two people. This was possible because it was built on the Flash platform.
Cheap and quick development means that underdogs like Maid Marian (the company behind Sherwood) can produce something profitable, and in theory it would mean they could try something new -- although they rarely do. Millions of people have played browser-based games, many of them Flash-based, but have you? If so, do they provide the basic gameplay you demand from an massively multiplayer virtual world, or were you left wondering why you even bothered?