I do sympathize with a desire to not have someone-dies-everyone-loses missions, partly for the above-raised reasons. We'll probably be able to figure something better out. Right now what we were most trying to avoid was giving players reasons to have a bunch of allies idle just inside the mission area so that they had as many "free tries" and/or runs-back-to-town as they wanted, where they would only get one try in single-player.
Perhaps a better option would be:
1) the boss/monster stat scaling inside the mission area keys off the total number of players that have ever been in the mission area (any chunk of it, not just the monster's chunk), so that if you have allies "waiting in the wings" that's fine but the stuff you're facing will have that much more health and you'll have to do more damage than you normally would
2) death in a mission area does not automatically cause mission failure, rather it increments the number of player deaths on that specific mission's object
3) when the number of player deaths >= the total number of players that have ever been in the mission area (the same number used for monster scaling), the mission is failed
4) picking a new character after dying in a mission area respawns you in the mission "staging area" so you don't get to go back to town and re-kit and whatnot (you can do that by abandoning the mission, going back to town, and restarting, but not while retaining mid-mission progress). This is how we're planning on handling death for the non-death=loss missions, anyhow.
5) when a mission is failed or abandoned, the player-death-count and mission-monster-scaling numbers are reset to zero, so the next time the mission is attempted it's a clean slate
As far as griefing, that kind of behavior generally cannot be prevented in the game design itself without compromising the design. People problems are almost always harder than technology problems. Rather, it would be better to let the server admin specify who had the authority to start a mission, to join a mission, or abandon a mission. Possibly letting the person starting a mission say who's allowed to join in to that specific mission too. That last one could be used by the game to let an individual player "abandon" a mission without abandoning it for everyone: the game would just not let that player back in unless the whole mission was restarted.