I think this is a very core problem that could be fixed, and has already been kind of started on already. so first why do people not join groups or post an lfm?
ineffectiveness of player or their toon (:P)
some of these we can fix, some we cannot. I would like to point out I think the new ransack mechanics help because instead of farming something once and being done you can go back to it in a day or two and farm it again, thus LFM's for good farms are seen slightly more frequently from what I have witnessed.
1) death penalty: is one of the few D&D mechanics still around, and for that reason alone I would not want to se it go, but from a gameplay standpoint people don't want to die, and the group should not be punished for one persons mistake. so possible solutions:
-remove it completely
-limit it to those characters that die (one person dies, they and only they get a -10% xp)
2) dungeon scaling: get rid of this, bringing more people should only ever be a good thing and encouraged. if it makes things easy, good, its an incentive, its supposed too. perhaps raise base difficulty a tad but do away with dungeon scaling. so possible solutions are:
-remove it completely
-increase base difficulty at all levels a bit (2-3 people) but remove scaling.
3) dungeon alert: this really needs to be tweaked, especially when some quests its unavoidable by design. but if skipping monsters is a problem then increase the reward for killing them or change quest design to require killing them (ala high road end quest) so possible solutions
-decrease base xp a tad, but increase killing bonus to give greater net xp overall make killing monsters more xp/min effective
4) no solution here :P
5) I think a bonus of 2% xp per party member (raids excluded maybe? I don't think they get run for xp so maybe a possible 24% bonus xp would be good?) would appeal enough to xp per min players to bother posting a group. I also think that a better teleport to the quest feature needs to be implemented so that when you join an LFM it will bestow the quest (if possible) and give you an option to teleport straight to the quest entrance.
As it stands a lot of people just don't like waiting for groups to fill, and even when they fill you have to wait around for people to wander their way to the quest.
conclusion: I think if some or all of these were implanted we would see a growth of LFM posting if bringing more people had no drain on the host and would only provide a benefit to quest completion. perhaps solo peeps would just want to post an LFM for a 12% xp bonus while they solo'd the quest :P at least we would get more groups.
P.S. I solo or group when I want and enjoy the game very much, this is not meant to improve my game directly ^_^