The Solo/Casual player can grind solo leves (from the Guild) to his/her hearts content with a cool story arc (somewhat different for each Class) that is unique to the Solo experience. He/She can also level all the way to cap without ever having partied if they so choose. Hopefully it was so much fun they want to do it again as another Class and get a different experience. Everyone would do these LeveQuests rank 1-10.
AT Rank 10 you can
choose to do Light Party Leves (2-4 players) These would be available in all Cities, esp Guilds, are harder and have cutscenes featuring all 2-4 party members (Smile..pose for ScreenShot). Hopefully it was so much fun that just doing them again with different people, as well as different Classes feels new.
At Rank 20 you can
choose to join Full Party Leves. These would mostly be available (or send you) deep into BeastMen territories, Dungeons, Defense Posts, etc., are very hard and require teamwork to win. The rewards are higher tho and encourage replay for the Drops (Gear/Weapons/synth items, etc.) The CutScene Reward is a nice KillShot video of the member that had the Killing Blow/Spell/DOT. Hopefully it was so much fun that these Leves can be endlessly replayed as different Classes and different PT's (and to get that coveted KillShot of Ifrit).