Well, to sum it up quickly:
+ Multiple clip edit is handy, although a bit of a misnomer. It doesn't let you edit multiple clips simultaneously, it just lets you view multiple clips, which you can edit one by one.
+ Groups within groups is very nice, and exactly what it says on the tin.
+ The new devices are great, although chances are high you're already covered in this area with VSTs.
+ Max for Live baked-in has made the M4L experience a lot smoother. I used to almost hold my breath every time I loaded a M4L device or a set containing one, but it really does seem more solid now. It's also great not having to update Max independently.
+ The colour coding in the browser is a super-lightweight but super-handy feature. I use to keep my go-to VSTs and native devices in groups.
+ Capture is very helpful for those occasions when you strike gold while just playing around without recording.
+ I think they've finally made dark themes usable. Kudos.
- The "simplified" automation view has made a lot of users frustrated. In a nutshell: if you like to add multiple automation lanes to a track, you're not going to love it, because Ableton have removed the collapse/expand automation lane button, so you're stuck with actually deleting the lane or seeing it. Also, if you are of the above mentioned inclination, you will also see all automation on all tracks when you are in automation view. So potentially a lot of automation lanes, then.
- The multiple clip thingy seems a little half baked, considering that users have been requesting multiple clip transpose forever.
- The "show all tracks" command (S), which collapses all tracks to their minimum width is a one way trip, ie. not a toggle.
A lot of people have complained that it doesn't feel like a .0 upgrade, while others point out that if you look at the price for a single VST, like Serum, you're getting a great wavetable synth plus some great FX units plus a lot of new features and improvements. I think that you'd be perfectly happy staying on Live 9, but once I had played with the beta for a while, I just didn't want to go back.