Assuming they mean Z=height, I'm not sure what the summary is talking about; you can most assuredly fall straight down (splat). Usually you aren't in a position to fly straight up, but the mechanics would allow it. You can't look completely straight up and possibly straight down; that is, or used to be common in FPS type games. The summary implies it is Doom-like 2.5D, but its true 3D; you can be at various heights and some maps are very 3 dimensional.
Solo play near impossible? There are spots where its a bad idea, but up to around level 80 its not hard to solo with a merc for most classes. The game has gradually gotten easier, and leveling faster.
As far as being out of date, that is part of the charm of it. They've updated a few zones to be more "modern" and often players like the older versions (Freeport, for example, once a sketchy port shantytown, now its a huge metropolis). Most people who want to play a modern game.. are playing something else.