1. In Act 1 while saving Cain from the skeletons, if you kill all of the skeletons then Cain will not talk to you and you can not progress.

2. Also in Act 1, if you manage to kill the cultist in the Khazra Den before he manages to say, "Kill him!" then Maghda will not appear and you can not progress. (Thanks Garfm)

3. In Act 2, if you go too fast after exiting the first time you go into Emperor Hakan's room, then sometimes you can leave Leah behind. She will then not move and you can not progress after that..

4. In Act 2, if you kill all of the enemies in the Wretched Pit, where you save Adrea, before the cutscene can end, then the screen will remain black and you can not progress.

5. In Act 3, if you leave enemies near the first two catapults as you run by them, there is a chance that their progress meters' will simply stop filling, so no progressing there. Garfm has said that the first catapult only locked when he left an elite pack near it, and it is mostly wise to stay at the 2nd catapult and kill the 4 devouring maws.

6. In Act 5, when taking Lorath to the ruins, similar to #3, Lorath can get stuck behind and he will not trigger the next quest, and you can not progress. This can be fixed by teleporting to town and back however.

7. Supposedly it is possible to one shot Diablo (and maybe other multi-phase bosses), but he will get stuck in his death animation.

8. If you talk to an NPC and hit escape at the exact same time, you will get an error and be thrown out of the game. Not exactly a soft-lock, but still not good.

9. It is possible for a siege rune to not drop, video example shown here: Possibly caused by knocking the enemy back immediately upon spawning.

If you know of any or find any not on this list, let me know and I will add it.