I'll keep this short... Although we kept the version numbering inside the 2.5.x series, 2.5.5 brings some new uber-cool features:
Smart Search integration for Joomla! 2.5
integration with anti-spam service StopForumSpam.com to stop real, human spam - available upon user sign up and for flagging spammers in comments
native support for jUpgrade (both versions) makes upgrading a Joomla! 1.5 site to 2.5 a piece of cake
integration with the Joomla! updater (v1.7+). Using the built-in extension updater in Joomla! you can check if there's a new K2 release and instantly apply the update
Community Builder integration for user profiles/pages
the K2 Media Manager has been upgraded and now supports image resizing as well! Just drag and drop images from your desktop inside the K2 Media Manager to upload them on your server, then resize and use them anywhere in your Joomla! site. The K2 Media Manager, already a full Joomla! media manager replacement, just got better!
(new) Added support for Falang, the JoomFish fork currently working on Joomla! 1.7 & 2.5 (JoomFish is not yet ready publicly for J2.5 - but that will come soon).
I'm also very proud to announce the the translation teams working on K2 language packs are now 52! Congratulations to all contributors and very big thank you from the K2 team! If you want to track the progress of translations or create a new transation team or join an existing one, check out the K2 project page on Transifex.
K2 version 2.5.5 will be released next week on Friday, March 2nd March 7th 2012. Sorry for the sort delay folks but we decided to add a couple of new features in this release.