With Mono, is has often happened that I have wanted to work on a fun new feature (say, C# 3) or implement a fun new class.
But most of the time, just when am about to do something fun for a change, I think "don't we have a lot of bugs out there to fix?", so I take a look at bugzilla and try to fix them, follow up on some bugs, try to produce test cases.
By the time am done, I have no energy left for the fun hack.
I need some "guilt-free" time, where I can indulge myself into doing some work that is completely pointless. But there is a fine balance between happy-fun-fun hacking, and making sure that Mono gets a great reputation.