Actually, the problem is not that we have too much solo content, it's that there's not enough party content. Most modern MMO gamers do not find old-school party grinding a la FFXI a desirable activity. Why would they when, like you said, they could do leves which not only yield exp, but also gil, item rewards, faction rep, and count toward achievements. Naked grinding doesn't give you any of that. However, if there were more low-mid level dungeons in this game (like Shposhae and Toto-Rak) that gave people incentive to group up and get not only exp, but also loot/gear, and achievements, you might see more partying.
The answer isn't to force partying on people, but to give them more incentive to do it.
Oh, and this power leveling garbage needs to go.