I don't know if this was mentioned in any recent patch notes, but if it was then I missed it, and if I managed to miss it then someone else might have missed it as well and could actually learn something from this post. Basically, Blizzard seems to have changed the way mob tagging and kill credit works - not everywhere, but at least for certain quest mobs.
I first noticed this when I was doing the daily quest Glop, Son of Glop for Therazane. If you've never done it, it requires you to talk to an NPC to activate a hostile elite whom you then have to chase all around a cave as he keeps running away from you. For a single kill this is a relatively lengthy affair, and I thought it was pretty stupid that Blizzard chose to make something like that into a daily of all things. If you entered the cave after someone else had just started the event you'd have to wait for a while, and if you then talked to the NPC after she respawned, someone else could still interfere and ninja-tag the actual quest mob. After a couple of days of doing this daily however, I noticed that some people seemed to like running along and fighting the elite with me even if they hadn't got the tag, and afterwards they exited the cave at the same time as me, apparently having completed the quest as well.
I went to investigate the matter on Wowhead and the comments there did indeed state that you didn't have to get the first hit in, as long as you just tagged along and helped to damage Glop, you should get kill credit as well. I tried this out for myself, and not counting a few times when the quest bugged out completely, it eventually worked.
Only a few days later I then heard a guildie mention that you didn't have to be the one to tag Problim in Tol Barad to get kill credit for the daily quest either. Curious, I went to find Problim myself and waited for someone else to attack him. It didn't take long for a random shadow priest to come along and engage him. As soon as he had dotted the mob up, I joined the fight - and incidentally saved his life, as he took a pretty bad beating and would have died without assistance. He thanked me afterwards, but the fact that my helping had indeed counted towards completing my quest was thanks enough really.
Since then I have spotted a note in the 4.0.6 PTR patch notes that says:
Kill credit for the Tol Barad bonus quest bosses will now be granted to players who have helped attack them, even if they are not in the same group. These bosses include Problim, Archmage Galus, Warden Silva, and Svarnos.
In the case of Problim this is obviously already live, but if the other quest bosses are still to come I'll welcome that as well, considering how awfully crowded it can get in the prison blocks just after a Tol Barad victory, with a dozen or more people spamming their AoEs in an attempt to tag the quest mobs for themselves immediately upon respawning.
I wonder if it is feasible to go even further and maybe apply this system to all quests that require you to kill a named mob. I really like it because it basically removes the "UI barrier" for grouping up. If we kill a mob together, we worked as a group whether the UI says that we're a party or not, and I like the idea of rewarding people accordingly. On the other hand there is some potential for abuse, depending on just how small the contribution has to be to get credit, as people could just follow someone else around and then throw a single dot on whatever the other person is fighting just to share the credit for it. I do think the benefits outweigh the potential risks in this case though.
Of course you then have to wonder whether this couldn't apply to other, more generic types of quests as well? Though I suppose as soon as you started to share loot things would get hairy...