I spent roughly 20 hours testing both Season 1 and 2 on the PTR and here is my comprehensive list for bugs I encountered:
1.) Random crashes in the Tower Act 1 (on entry to Level one or two) - happened random and only in single player for me (while playing hardcore)
2.) Names of died hardcore characters remain unchanged in Ladder (not sure if this is intended, in LoD Vanilla the names of died heroes were greyed out)
3.) De Seis spawned unpredictable
4.) Ancients were unable to spawn (Hell Mode, Softcore, Multiplayer, Season 2) - we had to re-open a game, then it worked
5.) Town portals did not open up (I was seeing the animation, the portal opened briefly but then vanished, scroll was used) - this happened in Hell mode and in “stressful” situations only (Throne room, room full of enemies). It also did not seem to be reproduceable (as the second portal opened up nicely.
6.) Quickcast and charge: sometimes i was unable to charge at the mousepointer. I primarily use charge as a movement skill and sometimes it was rather cast on an enemy nearby or the character did not charge at all.
7.) Portrait of A5 Frenzy barb showed A1 Merc
All the best