This game has a lot of action like Diablo, but is also a great "real" RPG in its own right. So it is definitely worth getting. But to answer your questions:

I have never had a crash to desktop in the full game (I never played the demo though). Make sure when you download the latest patch (1.34) after you buy it...it fixes some story bugs as well as technical stuff.

There really aren't three different skill sets...it's all one big skill set, like Morrowind. You choose how you want your character to progress. Almost everyone will focus on one area and have certain choice skills from another. The biggest difference between starting characters is their special skill and how their stats affect their vitality and mana.

If you hold the Control button down, it automatically targets the nearest creature to your mouse cursor. I assume this is also active in the demo. Kind of like holding Shift in Diablo to not move when playing a ranged character.

Nope, no multiplayer. But yes, like Morrowind, it is a strong single player game.

I think you will really like the game...I am sure that some of the more celebrated members of this community will have more things to say to convince you to get hooked!

Game on,

"There, beyond the bounds of your weak imagination Lie the noble towers of my city, bright and gold. Let me take you there, and show you a living story..." Genesis, "A Trick of the Tail" -- The Oneiromancer