He mentions that PHP’s MySQL Functions will become deprecated and what you need to do if your plugin contains them and he also explains the new symbolic links for plugins.
What he does not mention is that TinyMCE has been upgraded to 4.x and that has quite some implications for plugins that do anything with the editor.
Already back in January when 3.9 was still in alpha, I first read the news about the TinyMCE upgrade landing in WordPress 3.9 Core. I immediately tested how my SO Responsive Content plugin behaved and reported a tooltip issue that then was fixed.
Also new in WordPress 3.9 will be the Widgets Preview feature. Just like you can preview how a new theme will look, users will now also have a chance to see how a widget will look if they decide to add it to one of the widget areas of their site.
Therefore developers of widget plugins should at the very least check how the preview works and whether or not their widgets behave as intended.
As I of course need to eat my own dog food, I have just tested it with my SO Mobile Map Widget plugin and it works!
Andrew Nacin has kindly requested plugin developers to test their plugins and release an updated version with the “tested up to” version number adjusted to 3.9:
Plugin authors: Could you test your plugins against 3.9, and if they’re compatible, make sure they are marked as tested up to 3.9? It only takes a few minutes and this really helps make launch easier.
In the post he also refers to changes with HTML5 (Galleries and Captions) and Plupload.
So, if you’re a WordPress Plugin developer, please check your plugin(s) and adjust the date so your users know it is safe to update or install your plugin on their site!