Not a ton of details are available yet, but for some of the interface changes you can see the beta page here: https://www.dota2.com/reborn/part1/
I'll be glad to see Source 2 in use, and can't wait for Valve to count to 3 and give us something else built with it!
Announcement post: http://blog.dota2.com/2015/06/dota-2-reborn/

Super fun addition to dota2, I am digging the Warchasers mod at the moment.
This is something I still need to check out, but sounds awesome.
The custom games will really change how people play DoTA 2. The interface is much nicer and it runs better on lesser hardware which is a huge win for people on laptops.