1. Apple no longer allows you to upload updates to Dashboard widgets on their page. Even if I release an update, I cannot get the information onto Apple's pages. This means every Dashboard widget developer has to upload to a third-party site or maintain a site for the widget.
2. The development tools for Dashboard widgets no longer exist. At one point, Apple released Dashcode, a little development environment designed for making Dashboard widgets. That app no longer exists and even if one has an old version of it, Apple has stated it is not supported on Mavericks and that a developer needing it should stick to Mountain Lion.
3. Added security in Mac OS X requires apps to be digitally signed to be installed be default. However, Apple offers no way to sign Dashboard widgets. So anyone who tries to download and install a new widget is greeted with a message that says the widget “can’t be opened because it is from an unidentified developer.” The fix is to disable this check in the security preferences, something that some people just don't want to do.