multi-file uploading, one-click plugin upgrades, built-in galleries, customizable dashboard, salted passwords and cookie encryption, media library, a WYSIWYG that doesnâ€
t mess with your code, concurrent post editing protection, full-screen writing, and search that covers posts and pages.
There are a lot of features to post here, if you are interested you can check here. Some important things you will notice are Tag management, now it is easy to manage tag with out any plugins. Search improvements, now wordpress search searches both posts and pages, earlier it is used to search only posts. There is a automatic upgrade plugin function which might help lot of users, you need to provide FTP details for this thing to work and it depends on your hosting platform.
Other great feature is built in galleries, wordpress can handle galleries now with ease.
The upgrade process is usual like any other relase but you have to see all your plugins are compatible with it or not before upgrading. It is recommended that you take database backup before upgrading.