The new version promises enhanced user navigation and menu, automatic plugins update, built in gallery and multi file upload with progress bar, stronger password protection, shortcode API, a few database optimization and an inline documentation!
Inline documentation — the vast majority of the new code going into WordPress include inline documentation that explains the functions and documents their arguments. Now you won’t get lost trying to figure out what that freaky function call means!
Built-in galleries — A new shortcode that lets you to easily embed galleries by just putting [ gallery] (without the space) in your post. It’ll display all your thumbnails and captions and each will link each to a page where people can comment on the individual photos. Ain’t that cool!
Multi file upload with progress bar – Should I explain more? This is the biggest and most needed improvement which had always been on the top of my list. Great job guys.
Database optimization — The database table layout haven’t been changed, which is one of the reasons so many plugins work fine with 2.5. Only a few indices were added and the data length of some of the default fields were extended. You can see these changes here