There is a couple problems arising from the new system.
Now leeching (where players join and stand there letting others kill the boss) I think if they make it so you have to exit by the pentangle to gain xp, and maybe add a kick from game option it will stop. Now we don't need energy to join a game, it shouldn't be a big deal to be kicked and have to join a new one. Adding one xp per enemy killed will help and promote people to move.
Rushing the boss happens the most in the reaping. I think because he is mid map. Move him to the end by the pentangle and it should help. Another cause is that only defeating the boss gives you xp, again even one xp per enemy killed on a map should stop the rushing of the boss. another thing that will help is to toughen up the enemies on every multiplayer map to where it is impossible to solo the maps.
If someone wants to solo run, we have a selection for that. There should be a difference on difficulty between the two.
That's my thoughts on it and how to fix it. Thanks, I hope it helps.