Part 1:
https://www.dota2.com/reborn/part1
Part 2:
https://www.dota2.com/reborn/part2/
Big overhaul on the engine with revamps on pretty much everything that isn't related to the core gameplay (better replay system, voice chat on the dashboard, hero browser with cosmetic previews, etc)
There's even a project with making LoL using DotA2's custom modes
http://defenseofthelegends.tumblr.com/