Well, tanks (see, defender) are usually wanted very much in MMO's, due to nobody realy wanting to play'em.
From reading they're also beasts in PvP at later levels, although for the first while until they get their armor/hp/skills up they're lackluster at best. Typical tank there :3
Shadow walker seems like a typical gank/meless DPS type. But somebody who's played the alpha can likely give you more in-depth details on if that's right or not :3 Personally after seeing some images/videos of their claws I turned right off the class, nobody should use a fist weapon as big as they are, it just looks silly.
I really cants waint to see that new class they are working on. I have a huge things about guns man do I love long ranged attacks. I love rolling a healer however it seems that I always lose the what I call "stand alone complex" After always being a group just healing. I forget to how to fight sometimes.