This version follows 2.3. I don’t know why they skipped 2.4… maybe because of all the major changes in the admin area.

Yes, the admin area changed to allow less cluttering. However if you do a lot with wordpress, I would recommend you to test first before upgrading.

It could take a little of getting used to navigating the site, before you can do stuff with the same ease as you have done before.

From the security perspective, there are 3 good improvements:

Password strength indicator. A feature that is becoming more popular on many sites, very useful and encourages you to create better passwords.
I never make mine 100%, but at least mixing upper/lower case letters and adding some numbers can add a lot of strength to your password.

Salted passwords. This doesn’t mean you should pick easier password, however it is another layer of protection that a lot of other sites have already been using.

Secure Cookies. Also another layer of protection for HTTP request authentication. Cookies are now encrypted based on the protocol described in this paper.

One of the features I really like is the ability maximize the editor to allow full screen editing, and also the ability to upload multiple images to your post.
The editor has great improvements and new features. TinyMCE team has worked closely with WordPress on this release, so you’ll find more features that they use.

Another great feature, you can utilize a built-in gallery to group your photos.

The good news is that the database structure didn’t change. This can make your plugin transition easier for plugins that use the database, however you need to test each plugin first.

They added more indices to database tables to improve database performance.

To read more about all the main features, check out Matt’s post here.

Good luck upgrading!