Wow - a very 'us and them', aggressive attitude you've got there.
It's clear that for any app, site or OS to have mainstream success, it needs to have a well-designed interface, a good user experience. Apple, Google and others have all invested heavily here, and the big winners all have interfaces that are easy for a non-technical user to use.
If open source projects want to gain mainstream usage, they also need to invest in usability, and like any skill, there are people who have more ability and more experience. As these people are not necessarily programmers, it can be hard for them to contribute to open source processes that are centred around code contribution.
Regarding the us-and-them attitude, it's a vicious circle.
A person who is a self-defined designer often means it in opposition to coding. That means that they've defined themselves as someone who can't get anything done on their own. That works in a corporate environment, because they can talk somebody into letting them boss around developers on the payroll.
However, that leads to weird power dynamics and resentment on both sides. It also creates a situation where are lot of developers are discouraged from thinking about design, meaning they appreciate it less.
Where I depart from your analysis is the notion that open source projects really want to gain mainstream usage. I'm sure many would say they do, but I doubt many actually care enough to do something about it.
I think this is only going to get better in ways that erase the corporate line between designer and developer. E.g., developers who have some design skills doing more open-source work. Or designers buckling down and learning to submit patches like everybody else. If usability is the itch they want to scratch, they shouldn't wait for somebody else.
Otherwise, I think designers have to limit themselves to producing research and inspiration. E.g., doing well-supported usability studies on a popular open-source apps. Or producing design treatments as in a positive, here's-one-way-you-could-do-it spirit.
Many of these are very usable for their target audience:
Vim: adds such user-friendly features as syntax highlighting and WIMP while keeping an interface (vi) that is already well known to it's users, thus preventing them from having to learn a new set of commands from scratch
Emacs: Clearly the most programmable text-editor currently used. The ease of usability for extending it is the reason it is adopted.
mutt: synthesized interface from several already existing mail clients, most notably pine and elm
Apache: If you consider the user to be web developers, it (NCSA HTTPd which is what Apache grew out of) introduced CGI, which was a huge usability step for making dynamic web-pages.
Linux: Not sure. It's essentially a clone of SVR4; probably just the fact that among it's always had the best driver-support on commodity hardware is the win here.
Postfix: It's more usable than sendmail; not really a glowing review though.
X: Yeah, I can't think of a single good thing to say about X. I've heard that Motif was the "killer app" for it that kept it around, but I wasn't there so not sure.
All those you mentioned are highly configurable and accessible through the command line interface. For the target audience, they are highly usable. 'Traditional' (if we speak of the same thing) does not care if your users are general users or engineers.
It's clear that for any app, site or OS to have mainstream success, it needs to have a well-designed interface, a good user experience. Apple, Google and others have all invested heavily here, and the big winners all have interfaces that are easy for a non-technical user to use.
If open source projects want to gain mainstream usage, they also need to invest in usability, and like any skill, there are people who have more ability and more experience. As these people are not necessarily programmers, it can be hard for them to contribute to open source processes that are centred around code contribution.