I can see how people would stop playing this after a little while. Once you get to 45 pretty much all there is to do is start a new character, but there are only 3 classes. After leveling all three classes there isn't a whole lot to do.

I think that more quests would be a great idea! You could add quests that could be completed each day and people would do them. It would also be very beneficial to add some end-game content. Dungeons that only players at the level cap can get into would be a lot of fun. Especially if you made dungeons that require a group to have the right classes together in order to complete them. (Tanks, Healers, and Pure DPS)
You could implement some boss encounters that really require people to use their brains. i.e. A boss that could spawn some monsters that cast a spell to protect there master. You would have to down his little helpers before resuming DPS on the boss. I'm sure you guys can come up with many good ideas.

PVP areas would also be fantastic. A couple of maps that you could join and would automatically be put on either red or blue to level the playing field. These maps could be 8v8 or 10v10, maybe even 12v12.

Basically what I am saying is that more content should be available for level capped players to keep them playing and paying!

-Creepingtom