Obviously, you need to be bringing in a steady flow of new players to actually have any number of active ones. And since I've only been playing about a month I can give some insight about the first impressions of a new player.
Basically, in order to get more players, you need to improve on the tutorials and documentation. New players aren't going to stick around if they can't even figure out how to play the game. I found the character creation a bit intimidating, but not unbearable. However, I spent a couple weeks just figuring out how to drink a potion. And the crafting books, the information should be sorted by item (sabre, broadsword, and so on), not by task (shape, forge, and so on) and overlapping information should be merged or condensed. For instance, regardless of the material you make the shields out of, if the process for that type is the same, then why does it need a separate entry? And yes, there are crafting books available that are written by other players, but what happens when those players no longer play or if an update makes the information in those books incorrect? The fact that you even need these third party books should tell you there is a problem.
There are also little things that kill that immersion quality of role playing, things like displaying the weight of the furnace (do we need to know that) or floating herbs/plants and the torches in the Bronze Door Fortress where the flames all need to be raised up higher.
Some have said that the graphics are horrible, and to some degree that is the case, but more of that is the graphics errors like the rainbow effect in some areas, and the ability to see through the ground in others as well as some other anomalies. The engine definitely runs heavy and seems to run at the same speed on a single core machine with 2GB of RAM as it does on a dual core machine with 4GB of RAM. And that's in both Windows and Linux. Furthermore, certain sounds (which ones I can't be sure of) seem to crash the Windows client, especially in the area of the arena. This may be an OpenAL issue, but I don't know. But it looks like development for OpenAL on Windows stalled 3 years ago, and might be a problem later on, even if it isn't a problem now, so it might be a good idea to reevaluate that dependency or at least look into the actual opensource version, "OpenAL soft".
In the case of actual graphics, I don't think it's fair to compare PlaneShift to games like WoW. However, if you've ever played RuneScape you'd see just how bad graphics (along with every other aspect of a game) can be. So why is RuneScape so much more successful than PlaneShift? Most likely it all comes down to resources. They have an active development team and they continue to add custom content. The only other real advantage they have, aside from commercial marketing, is that there is no installation required as the game plays from within a browser. And while all of this may sound irrelevant, RuneScape usually has thousands of active players logged in at any given moment.
But success can't be measured on number of players alone, and despite it's quirks, I am enjoying the overall gameplay experience.