• The dashboard is now customizable. You are now able to choose what feeds show up! If you would rather other news sources – you can easily change the feeds by entering a new feed URL.
• The number of widgets you use is now unlimited.A lot of people wanted more than 9 RSS feed widgets, more than 9 text widgets, etc and now you can have as many as you would like.
• Managing media just got easier. There is now great built in support for a “media library”. No more using multiple plugins for uploads – you now have all that functionality built in.
• Updating your plugins just got easier. You can now update your plugins with a click of the button – but this means you will need to save your FTP user name and password and then trust that the updated plugin code is safe. While this is very handy and will save a lot of time for people not comfortable with FTP – the cost is security to some degree. I doubt I will use this personally – but I will also see how it plays out.
• Choose your own thumbnail sizes.You can now set the size of your thumbnails (small and medium size options). This setting is under “Miscellaneous”.
• Prototype library has been updated to version 1.6. If you are using any custom plugins that use Prototype or any custom template code that rely on Prototype – you might experience some issues if you are using an older version. Make sure everything you use is up to date. (I ran into this issue personally)
• Gravatar functionality is now part of WordPress – no more plugins needed. Here is a quick way to implelement gravatar support in WordPress 2.5+ comments: In your comments.php file you can add in gravatar functionality by simply adding:

  before : That is it! You could also add gravatars to posts by putting the same code into the post loop to have the gravatar show per post author. (The number – in this example 40 – is the size of the gravatar)

Make sure you check out the screencast that Matt created which highlights the new 2.5 features.