Well, PHP 5.5 still has some growing to do, I think. I know they removed a lot of functions (not saying they didn't replace them with better ones) in PHP 5.5. I'm not sure how that will relate to WordPress and Joomla! scripts, as well as other popular scripts.
There's also no ZendGuard for PHP 5.5 (unless one has been release recently) so that might break some things.
I agree that PHP's development is a little... something. They've got PHP 5.3 which is still technically supported (if not by popular choice), PHP 5.4 which isn't really getting new features added, PHP 5.5 which I'm not really sure what they're doing with it, and now they've started working on PHP 5.6. Having that many versions available just causes a lot of issue. A better choice might have been to retire PHP 5.3 before starting on PHP 5.5, and thereby limiting the number of different versions they have available. At least, that's my 2 cents.