The main point of 4.2.2 is compatibility with PHP 5.4. There are some Facebook bug fixes (Publish to Facebook stopped working unless you manually updated the SDK files) and some other minor bug fixes from 4.2.1.-
Full list (so far) is here-
http://www.vbulletin.com/forum/forum...-been-released
If you're happy with 4.2.1 and don't need PHP 5.4 support I wouldn't be overly concerned about upgrading immediately, especially not till it is final.