Basicly this in a nutshell. But from what we've seen so far in ARR (videos etc), it looks like there will be a huge variety of options to apeal to everyone.
Start: Fairly easy quests mixed with lore up to ?? level. (little reason to party, lots of tutorials, good grasp on core mechanics for new players)
Middle: More, very interesting quests mixed with exciting leveling dungeons (from Yoshi Letter), more reason to form parties, some new mechanics.
End: Party, Solo content