“[S]uccessful open source software projects tend to be at the developer-level, not the end-user level. E.g., successful open source projects have programming interfaces, not user interfaces. Apache, Perl, Python, gcc, PHP, the various SQL databases. The list of fantastic open source developer software is long.
The list of fantastic open source GUI software is short. This is not a function of chance.

This is dead-on. And I’ve been ranting about that for years. (That link is courtesy of the nifty Wayback Machine, because I’ve since lost the file that was part of a now-defunct version of 10RW.) I wish I could dig up my first rant about that, which was back in 1995-96 when someone on the FutureCulture mailing list was trying to convince me that the English department I taught in would be better off trashing our Windows lab in favor of Linux boxes. In 1996! Ugh!
Update [12:35pm]: I forgot I also wrote about the interest horizon and open source course management systems two years ago, as well.