The browser version is basically only "creative" mode, whereas the downloadable version randomly generates terrain with biomes such as desert, jungles, plains, tundra, etc.
I've yet to find a good server to play on -- seems like most of the ones I've run across are overdeveloped and not much fun to play on. My Minecraft experience has been mostly limited to single player, and a private server that my wife and I played on over LAN.
If you introduce kids to this game, watch out. You might never get the computer back!
My son found this game a couple weeks ago and is now obsessed with it. Right now he's playing the free, classic version and watching a lot of Youtube videos about the newer version. He's begging me to buy it for him. I'm tempted to buy it for him because I'd rather have him playing this than Black Ops or something similar. Is it worth $27?