The gameplay itself is very simple and easy to pick up. As for the graphics, they're very basic; let's just say you don't play Minecraft for the graphics. About the fun factor, this is where Minecraft really shines. As an open-ended game, you can literally spend hours just carving out a landscape, building houses, planting trees, digging tunnels, etc. Since you have to collect resources to build certain things, players will spend some time digging deep underground, where resources like coal, iron, gold, redstone, etc. lay. With the game worlds being enormous, exploration of the map (both above and underground) can take weeks (or months?).
There's also a multiplayer component to this game, as players can join the same world and build/destroy things on the same map. I've setup a 24/7 server for Minecraft. Since the world is constantly being saved, anything you build, change, etc. will be persistant (unless someone else destroys your creations ).
Server Address: y-corner.game-server.cc
Current Server Version: 1.4_01
Client Download
Here's a wiki on crafting certain items: http://www.minecraftwiki.net/wiki/Crafting
If you connect to the Y-Corner Minecraft Server, here's some commands that will come in handy, especially if you get stuck, which you will.
/help - Displays all available commands
/home - Warps you back to your 'home' location.
/spawn - Warps you back to the server spawn point.
/sethome - Sets your current home location
/playerlist - Lists all the current players on the server.
/item <itemid> <amount> - Creates an item and stores the specified amount in your inventory.
Build away!