Sometimes, when I click within a multi-line text-area, it gets the blue-bar styling to indicate focus, but it doesn't expand as expected; even once I start typing.

I've seen this once today (post deploy train) in the "Start a new topic" box, where clicking didn't expand the post-text-area. (But clicking outside, and then clicking within again, fixed it). I can't reproduce since then.
This error was also reported this morning (pre deploy train) at https://www.mediawiki.org/wiki/Topic:S9vtn34wgjhmnopw

Also, the 'Post a new message to "{{PAGENAME}}"' box is usually just one line tall when I start typing (Safari 6.2.2, Mac OS 10.8.5, Vector).

I've also seen this more frequently in the Reply to [topic] text-area, at the bottom of every topic. It sometimes doesn't expand (slightly-more-often, but still inconsistently). e.g.