Tera functions just fine as an action MMO, as do Vindictus, Dragon Nest and other action MMO's that have positional/timing/aiming requirements.
Sort of. You can solo all the dungeons (it's a lobby game), and some of the later bosses can be a challenge depending on what your setup is/how good you are. Beyond that there are raid bosses which are very difficult to do while solo if you are in the appropriate level range.