Directly from WordPress…
“Version 3.8 of WordPress, named “Parker” in honor of Charlie Parker, bebop innovator, is available for download or update in your WordPress dashboard. We hope you’ll think this is the most beautiful update yet.”
I don’t remember what the old version looked like so I don’t have a direct comparison. I also don’t remember having any problems other than I settings are not always in logical places (that’s my opinion. Yours may be different). It’s hard to go wrong with simplification though, unless you remove important items, which I don’t think they did.
It looks like all the changes are graphical so there isn’t much else to say. Read the official WordPress blog post and see for yourself.
I am interested to know what you think. Feel free to add your comments below.