I don't really know a lot about Minecraft other than it is a basic game that is designed to be very customizable. The graphics are horrible, but apparently that possibilities are vast and already 3 million people have purchased the beta client (which also pre-orders you the retail client on release).
A couple of guys that played AC and I played Darkfall with them have decided to start up a PvP Minecraft server with ideas taken from UO/AC/Darkfall. The server just went up about a week ago, and I believe they have much more to add as time goes on, but from the sound of it they are already having some random PvP fighting happening.
I plan on trying out and helping them with this experiment whenever I can find a little cash around to purchase a minecraft client. The website is:
http://www.savagerealms.net/ and I hope you won't mind me posting this link NH.