It is worth noting that mixing stable with unstable requires some knowledge and experience to tell the difference between a real bug, and a problem caused by invalid mix. Those "invalid mix" problems could be considered incomplete package dependencies, but in practice the assumption that the system is fully stable or fully ~arch and that all packages are fully updated makes things a bit simpler.
Now I'm going to try to answer the questions from Jeremy's post.
Well, maybe the "nothing really happens" part is the problem? It requires some effort to build a little community testing various packages, but it really pays off. In case of www-client/chromium and related packages, we have an active, growing community, and many of those people are using hard masked, and even 9999 ebuilds. Okay, poppler is a bit different (it's a library, it's not as "interesting" as a web browser), but have we tried getting more people interested in testing it? I think more people follow the Gentoo Planet than read random package.mask entries.
I'm not using ~arch daily, so I don't really know. However, I consider ~arch not being broken "all the time" a good thing. It has to be useable if we want people using it. It may be worth reading about Debian's experiences with testing.
Yes, I think overlays, if overused, may be hurting Gentoo, causing unnecessary fragmentation. I'm really glad to hear that XFCE team keeps things in the main tree. I'm using XFCE daily (stable system), and I like the end result of that.
No idea. I trust Zac's and portage team's judgment.
I'd say it depends on the contributors and visibility. If it's a random entry in package.mask, don't expect that people will "just" unmask it and start sending bug reports and patches. Similarly, people who can unmask things and are interested in what to unmask, following developers' blogs, etc, are more likely to submit useful bug reports.
Finally, let me quote a very important part of Diego's "Put it there and wait for users to break" isn't a valid QA method: