An MMO Universe is as big as the imagination of the developers who are designing new games. But the popularity of any MMO title is determined by the amount of people willing to play a particular game for longer than a few days.
I will ask why some MMO titles did not achieve the success which was expected, the answer could be very simple; lore, content, character progression were poor or too difficult to grasp -- I believe that in the case of 80% games vanishing from the market this is exact issue.
But it seems that there is whole group of games which did not achieve success regardless of being very decent products. They are somehow underplayed, even if hanging on in the market for long time. Still, regardless of having a dedicated but small player base, they could use some additional love treatment from MMO fans.
Let's take a look on 5 Underplayed MMO's You Should Try Out Now.