As a PvE-centric player, this installment really got me back into the game.
However, Nightfall really redressed this situation greatly. When I found out that now you had heroes that you could customize yourself and buff up like your own character, I realized that it would make some of the previously harder missions much easier to solo. In addition, the extra titles introduced at an earlier level of the game makes reaching the level 20 cap not quite as game-ending as it was before. Plus their was a new world to explore.
In terms of negatives, the plot is a bit weak if you like that. The addition of heroes did allow more characters into the overall story and allow a bit of character development, but it still feels disjoint and I never really identified with any of the story elements on an emotional level. It's still there just to maintain the pretenses of being a single player game.
In summary, this game is almost exactly what I'm looking for in an online game experience. I want challenge sometimes, I want to dominate sometimes and just kill things, I want to be able to collect things, and I want to feel like I can do those things with only a half an hour. Personally, I believe this installment really brought the game up a whole notch.