What I noticed is a decreasing quality of GEM released via GitHub compared with the overall quality of GEMS at RubyForge.
IMHO there are at least two major explanation for this behavior:
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Prior to GitHub the 99% of Rubyist was Subversion-dependent. You can say what you want about Subversion, but it is definitely more easy to use compared with Git and everybody is aware of the trunk/tags/branches layout. Then people started to move to Git. Just a super-limited slice of Subversion users started to use Git with the level of knowledge it should require and, what I noticed, is that people started to forgot about tags.
Once upon a time there were tags. In subversion people were used to release new version based on specific tags so that you can easily detect which version you installed and which was the stable branch.
Nowadays I see tons of libraries always under development in the Git master branch. No tags, no stable branches. In general, when libraries where released via RubyForge there was an higher level of attention to the deployment step.
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GitHub makes the publishing step no more a hassle. That said, you can easily publish a new GEM simply pushing the gemspec into your repository.
In my opinion this simplicity might can lead to a lower quality. More less-skilled developed started to distribute GEMS because it's as easy as generating a new project with Jeweler (or a similar library) and pushing a git repository. They didn't know much more about release management, backward compatibility, release bumps, release maintenance.
Often I came across an unfinished libraries packaged as a GEM just because the developer forgot to remote the .gemspec file. Each commit caused a new GEM to be build with no apparent coherence and consistence.
I'm absolutely in favor of the "release often" practice but when it makes sense. Git provides an excellent branch support, you don't need to clutter the master branch with tons of unrelated commits and releasing unfinished piece of code that you call libraries.
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Last but not least, what I probably hate the most is the unlimited duplication of the same GEM. When RubyForge was the unchallenged GEM source, it was quite easy to find and install a new project.
IMHO, GitHub introduced an unnecessary layer of complexity. First, you have GEM both available via rubyforge as mygem and via GitHub as username-mygem. You often need to spend time to figure out which GEM is the most updated and holds the master development.
Furthermore, some popular GEM was no longer updated on RubyForge and many people continues to use them just because RubyGems doesn't notify you about new versions. Easy to understand, if you installed coolgem release 1.2.4 and the same library is now available as superuser-coolgem (release 2.0), RubyGems is not clever enough to tell you a new update is available.
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Now it's time for a disclaimer.
I'm not saying GitHub users produces crappy GEMS compared with RubyForge. I'm a GitHub user and prior I was a RubyForge user as well. Thousands of GEMS successfully migrated from RubyForge to GitHub without leaving the end-user in the "which one" limbo.
The best example Rails, but I can mention many other GEMs including (but not limited to) Capistrano, Hpricot, RedCloth... All those libraries are now hosted on GitHub and if you carefully look at them you can easily recognize the same level of quality as before.
Last but not least, all those libraries continue to be released via RubyForge as the master source so that you don't need to reconfigure your environment to detect whether to install rails-rails or rails.
Also, the end-user is not affected by development decisions. Take Capistrano for instance. A couple of months ago Jamis announced the end of its commitment to the development. The community took charge of the development and moved the master repository from jamis/capistrano to capistrano/capistrano. What would happen if the GEM was released as jamis-capistrano? All the users would have to switch to the new GEM and the new repository with lot of hassle.
This scenario never arised because RubyForge was and continue to be the main Capistrano delivery hub.
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In conclusion, I unfortunately notices an overall decrease of GEM quality mainly caused by more people approaching Ruby and RubyGems without the necessary level of knowledge. The same apply to a large number of Rails plugins.
GitHub cannot be labelled as the culprit. When complex things become more easy and more people approach them without an underlying knowledge, it's normal that the quality can decrease because complexity is a natural selection process.
Anyway, there's still an excellent level of quality in the Ruby community. It's amazing to see how Ruby developers are committed to unit testing and other professional programming habits.