I can almost entirely agree with this post.
There are some distros that seem to just be pointless, but then they probably make sense for the people that have created them and who are we to judge lol...
Then there are some distros that are just not put together or maintained very well, but guess what... all distros started out like that... those that were around in the very early days of GNU/Linux will know that very well. Even distros like SuSE or RedHat were not great in the early days.
The problem with Ubuntu is not Ubuntu Core. I actually really like Ubuntu Core and the work Canonical is doing. Canonical's version of Ubuntu with Unity though has been hit after 10.04 though, and that isn't only a matter of Unity versus Gnome Shell, it's mostly a matter of repo hygiene in my opinion.
Any distro's downfall is a lack of packaging quality and repo maintenance. Even Fedora had a really weak couple of versions due to RedHat calling strange decisions and people lost their motivation to maintain Fedora. With Fc22, Fedora has proven that they have found a new motivation, and that a community distro can produce high quality maintenance and packaging even if they have to live in the shackles that were put on them by RedHat.
Another thing is that some distros are not meant to be "good" as in "the usual high quality expected of an open source release", because they are distros that serve only a specific development purpose. The benefit of open source is that experimentation in public is allowed. It's part of the reason why open source release software is so high quality, unequalled by anything non-open-source. It's is still a very true statement that in order to really test stuff, you have to test stuff till it breaks over and over again, and have as many people as possible try to break it.
And the last thing that in my opinion is true about GNU/Linux distros, is that they have been cross-pollinating a lot over the last 5 years or so, to an extent where there is much less difference between the major distro families as there used to be. And this is also typical for open source: everything is allowed, and in the end, in the most natural manner, without any coercion or manipulation, the highest quality solution will surface and gain momentum. A good example of this is X11: over the years, it has grown into a monster, a collection of extreme hacks and patches that just works. Everybody agrees for years now that it has to be replaced urgently... yet in open source, the replacement has to prove its worth first, and it's not just a matter of an executive decision to change things like in closed source... open source is very user-centric... what works is what works for the users... Wayland is only just starting to work after several years, and Mir doesn't work yet... so X11 is still the norm. It will only change when Wayland provides higher quality and better user experience than X11.