MMORPG.com has a description of the public quests of Warhammer Online. These are basically areas in which some event takes place, and everybody entering the area automatically gets the public quest to participate. The events can have multiple stages, and players can join and leave whenever they want. For example there could be an orc invasion, and when 100 orcs are killed, a big hill giant appears as the final assault. Players can participate first by killing orcs, then by all ganging up against the hill giant. Sounds interesting, and new for PvE, but if you look closer the system isn't so different from a PvP battleground in World of Warcraft. And that gives you a hint about what could be going wrong with this. Alterac Valley isn't exactly a shining example of player cooperation.
The fundamental problem is that it is very, very difficult, if not impossible, to exactly measure a players individual contribution to a large group effort. Software can't tell the difference between a player guarding a strategically valuable graveyard in AV, which just happens to be not under attack, and a slacker who parked his character at an out of the way graveyard and went for a coffee. In WoW both of these players would get the same reward, that is they receive reputation points for everything their faction does on the whole battleground, but no honor kills because they aren't close to any fighting. In the case of AV that reward structure leads to people rarely trying to defend anything, and both sides rushing past each other towards their target. Only if you are where the fighting is do you get honor kills, so doing something strategically more clever gets you less reward and thus isn't done.
In the public quests of WAR, how will rewards be distributed? If just being in the area when the 100 orcs are killed is enough to get credit for all kills, some people will always try to stand a bit behind and get full rewards for minimal effort. If rewards are proportional to damage dealt, damage dealing classes get more rewards than support classes. How much reward is a point of healing worth compared to a point of damage, and what about overhealing? How much is a taunt worth, or any other form of crowd control like sheeping? If you get credit for any kill where you just "participated", somebody doing AoE damage will get more rewards than a melee fighter. The list of problems goes on and on, there is simply no system where the reward is strictly equal to the effort put in, for all possible classes and actions.
This is why in current MMORPG you are only part of a PvE group if you got an invite to the group. So if somebody is a slacker and just trying to stand behind and still get a reward, the group leader can kick him out. That isn't the most logical thing from a story point of view: If Farmer Brown has a problem with wolves on his fields and wants 10 of them killed, and there are two players on that quest, why does it make a difference whether these two players are grouped or not? If they are grouped they just need to kill 10 wolves, each kill counting for both players, if they aren't grouped they need to kill 20 wolves. But from a point of playability this is necessary, because if each wolf killed would count for every player on the field, without needing a group invite, some players would just stand there doing nothing until other players did the quest for them.
If players were automatically grouped by just being in the same area, how would loot be distributed? Especially for big boss fights that is a big problem. The fact that every guild in World of Warcraft has a different system to distribute raid loot proves that there isn't one obvious fair system. Imagine in a WAR public quest the final event boss drops an epic item, everybody around is allowed to roll for it, and the guy who wins the epic is some low-level rogue who just stood close enough to the combat being stealthed and not contributing or being in danger at all. A guild can enforce loot distribution rules by kicking out ninja-looters. But if everybody who turns up is automatically part of the raid group and can't get kicked out, then how do you prevent abuse?
If somebody came up with a brilliant solution on how to prevent abuse, that could be very beneficial to the social cohesion of MMORPGs. Right now, if you are hunting some type of mob which isn't overly abundant, and you see another player in the vicinity, you inwardly moan and are unhappy about seeing him. Other player means competition in who gets the mobs tagged first, and an increased time to finish your quest. But few people are willing to group with a total stranger for just 5 minutes to kill 10 wolves. Wouldn't it be nice if any player on the same quest as you would automatically be an ally, and not a nuisance?