Lets dive straight in here, Joomla! 2.5.28 is due out on the 10th December 2014 and this will be the final release of the Joomla! 2.5.x series.
Why does this effective you? quite simply security and performance. Joomla! are constantly improving and updating Joomla! Although no all updates are smooth, they are set to improve and not only that the foremost issue here to us is security. As Joomla! 2.5.28 will be the last final stable release, you won't be getting anymore updates. This is why it is important now to upgrade to Joomla! 3.x.