I actually bought magicka and found it unplayable. No, really, it doesn't even start. But that sure sounds nice, since i've heard about the awesome system in that game.
I've heard the developers are working like crazy to fix it.
http://forums.steampowered.com/forums/showthread.php?t=1754205
Edit ---> Apparently the game uses XNA instead of directX, so you need to install that together with the .NET framework. (from the microsoft website)
It'll probably fix your inability to play it.
Those guys remind me of Bethesda in the good 'ol day's, lol.
You have eight buttons which allows you to cast over a hundred different spells and in multi-player the magic beams can cross creating combo's or making the spells backfire and magic effects can interact with the environment.
It's has some really great ideas, such a shame they're having so many troubles with the release.