My frustration with open-source: Documentation (or lack thereof)
The problem I find, however, with a lot of open-source software is the complete lack or reasonable documentation. That drives me nuts, although I'm not entirely surprised. It's one thing to give away and share your work, but that scenario doesn't exactly provide a ton of incentive to document it properly.
NUnit ended up being useful to me I think because it was in use at a project I was on, and it essentially has its own book. Most stuff I've encountered doesn't have that luxury.
I'm not suggesting even for a moment that the world would be a better place without these projects, it's just that the price of entry is kind of high for something that is monetarily free.