Sorry if there is some repetition here, this thread is growing too fast to keep up with, but here goes.
Monsters just "going home" after they're zoned or their target(s) leave/die/etc. doesn't sit very well with me, I agree with the person who said that this would make a lot of things much too easy, and it also shouldn't really be needed (I do agree that making monsters go back to their start points should be sped up a bit. So, they'll still aggro people, but they will also not just hang out at a zone line for no apparent reason (they should return as quickly as they chase after a person, not at the pace of their normal shuffling about)). Then, make it so a mob won't aggro or link for *several seconds* after it loses its target. Basically, just a little longer than it would take for the mob to walk out of aggro range of anyone it was trained to. This won't entirely solve the problem, but if spawn locations for the NMs were moved out of the zone's pathways (into their own chambers or at least provide a side area that is out of aggro range from anything coming back from the zone) it becomes very hard to MPK people fighting an NM (because anything you zone to them will walk back out of range before it has a chance to join in).
For BST stuff, make it so that a BST pet cannot aggro until it is both at its start point and at full health. By the same token, the BST cannot charm a new pet until the old pet is back to its start point and at full health (this makes things a little harder on BSTs because they can't juggle 3 or 4 pets, but if you make pet food level-dependant and make it around 4x as effective and usable about twice as often, they will get similar "total pet hp" during a fight. It will also give a real reason for using the lower-level foods (because, well, you have to). This keeps BSTs from MPKing, reduces the difficulties that arise when a BST is killing in a popular zone, and makes a somewhat neglected ability/line of items more important and more attractive for crafters.
For AOE... much as I like the idea that AOE be AOE and if you're smart you stay out of the other party's way, the grief potential from things like bomb tosses (let a gob hit you next to a party and wait until it blows them up), mandra sleeps (grab a mandy and go chase parties who are fighting gobs around so their mages are slept a lot), and worst of all, skeleton blood sabres is huge. Blood sabre is extra bad because it works both ways... someone can drag a bone mob over to a party they want to kill and let it drain from them over and over. Or, if a party (or even an individual) you want to kill is fighting a bone-type mob, just get a bunch of folks to stand near them so it keeps healing to full (though, this is usually a stupid-people problem rather than a griefing technique). So, AOE from *regular monsters* should be enemy-only (person who currently has aggro, plus any other party members, plus any alliance members, plus anyone who has cast a spell on any of the above).
So, the actual suggestions:
Suggestion: Monsters that lose their target immediately begin to return to their 'home points' at running speed. They can aggro (if they are aggressive) but will not link (note- mobs in a train should be tied together until each reaches their start point. So, a crawler train returning from the nest entrance won't link in on someone fighting a crawler that was uninvolved but if a player does something to get aggro from a member of the pack, they get aggro from the pack as normal). Additionally a monster which is currently engaged should only be able to aggro the people engaging it (the 'enemy' from above, including helpers). This way a train of non-aggressive but linking mobs won't link in on a party fighting something in their family while they work their way back just because the train's target got a tiny bit of hate from running back. It also fixes the problem of people claiming a slept add, because as soon as someone else hits it all hate towards the origional party is removed.
Suggestion: BST Pets are non-aggro (regardless of normal behavior) until they are back at their start point and healed up (they return just like a mob that lost its target, at full speed). BSTs cannot claim another pet during this time (so you can't have them charm/drag/release all the dangerous stuff in a zone to make something unintentionally easy) but their ability to complete a fight with a single pet is greatly increased by making food a much more effective tool.
Suggetion: AOE from a normal mob only affects 'enemies.' This way you can't drag a monster to a party and let it continually AOE them to death. It also prevents people from getting you killed when you are trying to save their dumb butts from the skeleton they got aggro from, and it allows NM and HNM AOE attacks (which are intended to be big and scary for everyone and should be for the reasons others have described) to still deter people from getting too close to a fight in progress.
Suggestion: a bound monster will only attack 'enemies,' and not anything in reach. Additionally, when its target list is cleared it becomes unbound (and thus runs home before it links in on the people it is next to).
That *should* largely clean up the problem of intentional MPKs and help with zone-line congestion as well (if people want to actually fight *at* zone lines they should have to worry about things being brought in from time to time, that is part of the tradeoff of having a zone right nearby; however a zone should not shut down for a half hour because someone dragged a bunch of stuff up from deep inside).
A couple unrelated suggestions: Player AOE should behave like monster AOE- it should only affect mobs which currently hold hate towards the party (or have helped a mob with hate towards the party). The AOE spells don't get all that much use in a lot of areas beyond sleepga and horde lullaby because of the danger of links, usually by monsters *not* in the family of the one you are fighting (lizards in yhoator, caveberries in the nest, etc.). There really isn't a big negative to not using -ga spells so there is really no reason to make them so potentially dangerous. The single-target damage/mp is usually lower than a single-target spell so it isn't like BLMs would be getting some huge boost to their potential damage, and might even make it more common for a party to take on more than one mob at a time (because you can blast a couple at once without worrying about bringing in anything that wouldn't have otherwise added in). It also doesn't remove the danger of using AOE spells in an add situation, since a slept add would have hate towards the party and would thus still be hit.
One of my biggest pet-peeves right now (and a rather hot topic in some circles) is the current pandemic of power-leveling. Personally I feel that it is both against the spirit of the game (why work with a group of people when you can have one person follow you around and cure you all the time) and against the community's best interests (parties don't learn what is realistic so when someone who is used to being power-leveled joins an otherwise serious party they either have no clue what they really need to be doing, or they have wildly inflated ideas of what is possible (or worse, both). So, in my opinion, if an outside player helps you during a fight (or helps your party), the XP you get should be cut in half and there should be no chain bonus. To prevent this from becoming a griefing point, allow the playes to set a condition which will prevent outside aid (and allow party leaders to set party- and alliance- wide conditions) while the player (or party, or alliance) has a mob claimed. This would only count while there is actually a 'red' mob (alliance-claimed mobs need to show up red, rather than purple, btw), and could be overridden by a new "call for aid" command (which would put out a /say notice but no icon on anyone's compass and wouldn't be a /shout-range message). Calling for help, or disengaging, would obviously allow outside help as normal.
(but, best to stick to the topic at hand, a PL debate could derail this thread but it is something that I think really needs to be reconsidered by SE, as they have been continually adding in content with which powerleveling just doesn't fit)