I have just switched from REL_1_16 to trunk. I have the CodeReview extension installed, and have a bunch of local CSS rules (in [[MediaWiki:Common.css]]) which customise the styling of the CodeReview output.
In 1.16, before the resource loader was added, the load order for CSS stylesheets was extensions, then core, then user styles.
In trunk, this appears to have changed to be core, user styles, extensions.
The correct order shold be core, extensions, user styles.
The current situation means that my local styles are being over-ridden by the default CodeReview stylings, and there is no way to get round this - I can't even use !important, as the CodeReview styles are already using this!