Definitely. Pre-4.0 you could "steal" another guild's raid ID by running with them, then entering the instance with your own group, clearing it while the guys that started the raid are offline so that by the time they log on to finish the raid, their ID had already killed all bosses so they had to wait until the next reset. There recently was a bit of a contreoversy because someone stole a server first kill for Madness of Deathwing by using the personal raid ID system implemented with 4.0. No system is perfect, really.
This kinda depends. If it was really out of the way (like killing the Chief Architect guy in Blackrock Depths when your group was there to kill the Emperor on the opposite side of the dungeon), I can understand.My brief foray ran me into some which you see already here: do that quest/objective/etc some other time we dont have time
This is more a result of the lack of a way to shard stuff that drops into sellable items. At least in WoW you could get enchanting materials from drops no one wanted, provided you had an enchanter in your group.Alot of guarenteed stuff is vendor trash- etc etc etc.
You could implement something similar. Maybe have the group leader receive a "Volatile Materia Assimilator" upon entering a dungeon, which can be used to break down any drops no one wants/needs to materia or dark matter or something. The assimilator would "break" after exiting the dungeon, forcing you to get a new one upon re-entry.