They've both managed to maintain long-running virtual communities.
The ability for anyone to create is powerful, as recent games like minecraft have shown. Second Life just never kept up with technology, and now the best ways to create content for it involve the use of modeling software with a high barrier to entry. (Of course, it's terrible in many other ways, but that's a separate discussion).
If someone can build a virtual world platform from the ground up, with tightly integrated content creation that is intuitive (maybe something like sketchup), and all in-world, it would be very powerful.
I've actually been wanting to build just such a platform for a long time, but learning 3d graphics isn't the easiest thing out there :)