I've been running in to bugs where my interactions with mobs and quest givers can be drastically affected by other players. Some of these bugs are merely annoying. For example while trying to take the palisades outside Camlorn the Orc I'm supposed to meet with was at the wrong end of the field and when I went to talk to her she said we'd already broken thru. However, when I went to the next area the game realized I hadn't fought a single werewolf mob and reset the quest. No biggie, as a vampire I take as my solemn duty to slay as many fur-bearing varmints as possible.
But not all the interactions are benign. Later on in the Fall of Faolchu quest I needed to escort one of the guards around to light the signal fires. I cleared out the south tower with no problem, then headed to the west tower. As I got close I see an enemy mage and nail it with my shards but it was already in battle with someone else doing the quest. When I got to the tower the guard said immediately she had lit the signal fire and the quest updated to show 2 of the 3 towers were done. Cool, I thought, that was easy. Then I headed to the eastern tower and couldn't proceed. The arrow for the quest was there but I couldn't click on the door. Logging out and back in didn't reset it. I had to abandon the quest and do it over. Again, not the end of the world, I got to send more werewolves to their final reward in Hircine's fields but still a broken bit of functionality.
What matters though is it is a fundamental piece of functionality -- after all this is supposedly a multiplayer game. If we can't interact on the most basic of levels, killing monsters, without it messing up our game play I don't see how ZOS can get more complex interactions like PvP ever working right. I get the feeling that the underlying plumbing in this game isn't put together right, that they took the single player code base, slapped it on top of some multiplayer framework and have spent two years with baling wire, duct tape, and used bubble gum trying to patch all the problems. If that's the case then I seriously doubt they will ever get the systemic bugs worked out of the game.