I just upgraded to WordPress 4.0 on one of my private sites. So far the experience is a bit mixed, since I at the same time also upgraded the outdated themes and at least the Twenty Ten theme seems to be slightly broken somehow.
What I note after the upgrade is that images, especially in newer posts are incorrectly placed (typically moved from centered to right side) and incorrectly sized (smaller). Strangely it only seems to affect newer posts, made somewhere around the time of the previous 3.9.x version, but not older posts.
Fortunately though fixing the issue seems easy enough, just go into edit mode on each post and click “center” on each image. That’s all, but still with a lot of posts there’s still some way to go
Beyond that I’m not seeing much visible changes yet, which of course is good, since that only means added functionality to be dicovered but nothing really broken or big changes to start reading up on right away. Though I think I will still wait a little while before updating my main homepage since it uses a very customized theme.
Binero also have a nice review of news and updated feratures worth reading