Joomla 2.5 is changed a lot if we compare it to Joomla 1.5. There are many features are added to it. Here you can see a list of new features available: Migrate to Joomla 2.5
here the New Features Of Joomla 2.5 as described at http://www.joomladevs.com/Joomla-25-Migration-Service - Joomla Smart Search You will be able to do search through your content in a smart manner and it will also come up with Search Suggestions like Google. - Multi Database Support It will be possible to run Joomla on MySQL as well as on MSSQL(Microsoft SQL) - More choice for Offline Modes You will be able to choose a custom message for Offline Mode and also use image. - User Registration With Captcha It will be very useful thing to stop spam registrations. Joomla 2.5 will have Captcha powered Registration Forms. - Mobile Enhancements Default Beez template will come up with mobile enhancements. - Multilanguage Support Now, there will not be any need of using any 3rd party component for Multi Languages. Default Joomla installation will have Multi Language support in Joomla 2.5. - Subcategory Filter Category Filters were there but now in Joomla 2.5, sub-category filters are also added so you can find articles of a sub-category also.