From Diablo Wiki
Diablo 3 version 2 (or D3v2) refers to the radically-updated (and improved, in the opinion of most) version of the game that debuted in February 2014 with Patch 2.0.1. The game state prior to this patch is often referred to as Diablo 3 vanilla.
D3v2 introduced numerous significant game changes, including Loot 2.0, Paragon 2.0, major rebalances to to many of the character skills, an overhaul of the combat engine, clan and community support on Battle.net, and much more. All of these system changes (and many more) are also present in the Reaper of Souls expansion pack.
D3v2 was a controversial undertaking and many fans lamented the changes before they were able to test them. That opinion shifted considerably once the patch was available on the PTR, and after two months of testing most players were in favor of the game changes/improvements. 
What did you think of Diablo 3 v2.0.1 on the PTR?
Total Voters: 2,026
- 1) I played on the PTR and thought it was a big improvement. (44%, 884 Votes)
- 5) I didn't try it out on the PTR but think the changes look good. (28%, 561 Votes)
- 2) I played on the PTR and thought it was a little better. (12%, 251 Votes)
- 7) No opinion / not sure. (7%, 133 Votes)
- 3) I played on the PTR and thought it was meh. (5%, 111 Votes)
- 4) I played on the PTR and thought it was terrible. (2%, 44 Votes)
- 6) I didn't try it out on the PTR but think the changes look bad. (2%, 42 Votes)
The fan enthusiasm continued post release and saw many more players active during the weeks leading up to Reaper of Soul's release on March 25, 2014.
The version 2 paradigm was carried over into Reaper of Souls and has been modified and enhanced in many patches subsequently, all of which move further away from the original launch version of the game. In 2014 at a Blizzcon panel, the developers discussed the major changes they'd made to many of Diablo 3's original features, and why they felt these changes were big improvements. That panel was expanded in a very self-critical presentation by Game Director Josh Mosqueira at GDC 2015.