To solve some of these problems move xp away from power leveling and grinding and boring shit like that and make it about dungeons. By dungeons i mean things sort of like mob arena's (for those who remember them), but instead of waves spawning and a boss at the end, have a group move through the dungeon, clearing mobs and boss mobs to get a bunch of xp along the way and at the end. (I know that this has been talked about being done, this isn't even my idea I'm just clarify what i mean by dungeons). Work to push through dungeons and people wouldn't want to tear their hair out leveling because leveling would be fun and somewhat social, because you could do it with friends.
Additionally, maybe if when people buy boosts, it is a passive xp gain that was low but took effect over a really long period of time, like a month or something. The combination of not feeling compelled to level whenever there is a boost and refusing to level when there is not one and dungeons could possible fix the current state of leveling which (at least for me) is power leveling until my eyes fall out when there is a boost, and immediately logging off when it ends because i can't bear to look at mine craft any longer. Also if there were dungeons that were made for noobs, that would supply you with gear inside of the dungeon (like an ma), that would make getting into the game and becoming a higher level far less painful for new players. The lower leveled dungeons would give less xp and have easier mobs/whatever ur fighting. At this point I'm just rambling, but whatever this is my 2cents.