I believe that the success of Rails is ultimately a success in marketing. DHH presented rails to the ruby community delicately (I was following the ruby mailing lists in fall 2003). His website, the Rails site and most of his language have a level of polish that has sold the project to the point that it has taken on a life of it's own.
If you really want Paste to compete I think you need to spend a bunch of energy on marketing. Pay a designer to develop a nifty logo/template, etc.
I thinks its the combination of success in marketing and the technical excellence to back it up. If it were simply good marketing hype, no one would stick around after their first look.
There are plenty of web frameworks with a high degree of technical excellence. Sadly, it is the marketing that pushes an excellent framework to achieve social success.