After enabling the Akismet plugin for WordPress the other day (which I will say now, is superb) I thought that I would investigate what others there were. I went to the WordPress Plugin DB and found over 1000. Having now read most of the one line descriptions I selected only one that I thought would be useful—Self Comment Notification Filter which stops you having to moderate your own comments. In the case of 99% of the others I still have absolutely no idea what they do or why I should want them.
And that, I think is the problem with the Plugin and Extension system; the same is true of Firefox and Thunderbird only worse because the users tend to be less technically minded. It is neat that the products are stripped down and functional with no excess baggage that you never use (unlike some other products we could name). But there is something missing from the mechanism for extensions. There is no quality control so you have to try them and see if they work; there is little version control so they may not be compatible with the next release of the base product if the author has lost interest; and there is no good advertising.
What seems to be needed is an organised reputation system, probably managed by the base product owners. To get onto the list you should have to provide a minimal of description, function, documentation and screen shots. You should need to commit to a period of support, and there should be an organised classification (to identify potential users) and rating system (to determine quality).
For WordPress the Plugin Database meets some of these requirements and for Firefox/Thunderbird the Firefox Addons site and the Mozdev extension room offer similar services, but they all lack that something that would encourage the general user to look for and find the useful tools.
Themes and skins suffer from a similar problem unless promoted by the base product owners—even with a product like WordPress, that is only likely to be used by the technically savvy, it is remarkable how many are set up using the default theme.