I can say from project manager's point of view that it takes MORE time to prepare MediaWiki developer that Drupal/Joomla developer out of PHP/js programmer.
I've had employees and students, and many of them complains that there is no programming documentation in MediaWiki. This is not true, we have lots of docs for almost every hook, class and function. However I can admit that there is not much materials and article to glue together these specs.
In other words we have good reference manual but bad tutorial. The only thing that I can call 'a tutorial' is this one. It's a big mess now and it needs improvement:
- first things first. i18n, aliases, $wgExtensionCredits and defining configuration variables are NOT the most important parts of the extension
- Where are the BIG ideas? First big idea is that extensions use hooks mechanism. Second big idea is that extensions writes and reads directly from database, unlike plugins in other CMS. Third big idea is that there are two APIs - the PHP classes like Request, WikiPage etc and well-documented client API.
- Documenting the PHP API: Hooks and PHP Classes. I really don't know how to better do that :(