I know there have been discussions of this before, I've just read a number of them from last year, but I just needed to bring it up again because it's driving me crazy!
A) While I do like to be able to do things with other random non-grouped players who happen to be in the same area, sometimes (but not always) if I'm on a quest and am simply near something I'm supposed to fight, someone else killing it will somehow count for me, even though we're not grouped. I've been notified that some objective was completed before I even SAW what I was supposed to be doing, which makes the quest feel rather disjointed and takes away from the feeling of accomplishment. Yes it's nice to be able to work together on tough bosses sometimes without having to specifically group with someone first, but... when I've barely entered the area and haven't even drawn my weapon yet, it really doesn't make sense to have it count for me. It just makes things a little confusing when your objectives switch even though you aren't aware of actually having done something. On the other hand, I've been in fights in dungeons with other random players where I have ended up distracting the lesser enemies that are standing with a Boss, while someone else focuses on taking him down, but in that instance if I somehow haven't gotten a single actual hit on the boss then I won't be given the achievement. This just seems so inconsistent.
If the game can tell I never actually hit the dungeon boss, why can't it tell that I never hit a quest related target?
B ) It's even worse when you actually
are grouped but sometimes you each have to do everything individually anyways, and other times once one person does it it counts for everyone.
1) The worst problem is when activating something teleports you somewhere else, but activating it counts for everyone in the group resulting in one player being advanced in the quest while everyone else is stuck in the previous area with a now un-activatable object, or NPC with none of the correct dialogue available, and have to take a long, round-about way to meet up with each other again. When doing the quest at Firsthold to close the portals to the Deadlands, my husband got stranded in Oblivion after his grouped friend destroyed the glowing orb at the end of the little cave, luckilly he finally figured out that he could get back to Tamriel by going back to the beginning of the cave, but it was very confusing. Likewise when I was doing the quests at the Tormented Spire with a friend, I made it to the end of the cave while she kept getting thrown back, but as soon as I'd done what we were supposed to do back there it told her to talk to Mavos, who then automatically teleported her back to where we were supposed to go, and when I finally got back out of the cave and tried to talk to him he had no dialogue for me and I had no idea what to do and had to find my own way back down and ask my friend to explain what he'd said and where he'd sent her. It was very annoying.
2) Even when it doesn't leave you stuck somewhere, it's very frustrating when you're in dialogue with an NPC and one of your group members exits the conversation first and suddenly everyone else is forced out of the conversation because the NPC has run off to do something and you're left not knowing what they were about to tell you or ask you to do. As someone who actually wants to role-play and be IN the story, rather than just grabbing the next objective, I find it terribly disorienting and disappointing when I don't have the chance to hear an NPC actually tell me why they want me to do this thing or explain the Story that's going on just because a group member reads faster than me or doesn't care about the story.
3) Sometimes it seems overzealous in counting things for everyone, as at Ezduiin, each time I or my other group member released a spirit it counted that spirit twice, once for me and once for her, so instead of needing to release 10 in total between the two of us, we only had to do 5. As convenient as that may be, it's terribly inconsistent with other cases in which we both had to go do everything individually - such as later in that same area where we both had to solve the puzzle of the constellations individually in order to complete the quest, rather than it counting for us both. At Cath Bedraud we were able to collectively gather talismans from the necromancers (it didn't double up the count that time as at Ezduiin though), but we had to protect Gabrielle Benele twice as she cast the spell twice, once for each of us before we could both enter the crypt.
It's like the game just can't decide whether we're really doing this as a team or not.
4) It can also be a little frustrating when group members pop in and out of existence when you pass by an area where one of you has completed some quest that changes the environment, while the others have not. I understand the alternate realities could be hard to handle but it seems like it would make sense to have it either default to the state in which the group leader sees it, or to the state of the person who is the least far along in that area so that you could still actually help a group member who hadn't completed something you'd already done. This is especially true when it comes to doing quests where something doesn't count for you both, and one of you completes it first and then just disappears into the new reality while the other is left wondering where you went.
5) And then there's the issue of some quests being able to be done as a group up to a certain point and then suddenly it takes you into a solo instance that you may not be prepared to deal with alone, or at least where you have to switch around your tactic, not to mention the sudden loss of camaraderie. It's disappointing when you were planning to hang out with your friends and then end up spending a bunch of that time solo-ing when if you'd known before starting that quest you could have just done that on your own some other time and spent your time actually with your friends doing something else that you could do together.
The confusing inconsistencies in how the game handles groups really detracts from the enjoyment of playing in a group, which seems like a major problem for a game that's supposed to be designed for multiple players. I really hope to see some improvement on this in the future, but given that the same problems have been ranted about a year ago I'm not going to hold my breath

. As it is I'm seriously feeling like I don't want to do anything grouped any more if I can help it because I'm rather sick of the confusion of constantly having to figure out if we're still on the same stage of the quest or clarify what's going on and who still needs to do what, or why someone doesn't have the option anymore to do something.
-Marcusstratus's wife