Ori.livneh asks I noticed that PagedTiffHandler isn't explicitly versioned. Why?
Basically, there have been no new features or major improvements (that would justify a new version) after we handed it over to the foundation for deployment on WMF sites. So I think, for the time being, the tagging related to MW releases will suffice.
However, you hit the mark with you question: there is no consistent versioning system for extensions, nor any quality assurance on a regular basis. That's an issue we need to address! Our proposal's scope, for the longer term future, includes to tackle that one. There are models out there for assessing extensions, take for example Typo3 (seeme to be more centralized) or Wordpress (more community driven). I think we should not reinvent the wheel, but take a close look on current successful models and adapt them to our community's needs.
One possible solution could be to require Selenium tests to become "certified". They are easy to record and test the workings of the extension on a user interaction level. But at this stage, that's just an idea I personally dally with ;)