Well, I'm in the throes of trying to contribute to core for the first time, and I have to say, it's a painful experience. Your concepts and code are suddenly thrown open to pretty fierce scrutiny by the community at large.

When you're making your own module no one really cares what you do, as long as it doesn't compromise the security or operation of other modules. Not so with core. There can be some pretty harsh debate around what can and cannot go in to the framework.