Here some Points why I think Minecraft had such an impact:



#1 Accessibility: Its a browserbased 3D game
-virtually anyone (with Java installed) can start the game instantly without
installation.
This is a huge point, by reducing the "costs" for
users to give it a short try.
Its exactly the opposite to making an Indygame as a 800MB installation file...
Also its 3D, giving it a huge athmospheric advantage over many 2D Flash games.
I doubt that it would have worked nearly as well
as a 2D tilebased or isogame.



#2 User-Content Creation: It offers every user (without required any special skills ) to create interesting things.

This is something not totally new, but often underestimated.
Games like "the Sims" live from the core concept to have
people create their own world, ideas and impressions.
The more freedom the mechanics give you -while keeping it easy
to design- the more fun this concept is.
Also its is a sandbox game, that does not force any goal
or timelimit on the player. Anyone can play it the way they like it, as there is no "right" way to play it, or ultimate goal
to reach.


#3 realtime AND asyncronous Multiplayer

While the success of the concept of realtime Multiplayer is clear (WoW, Countersrtike...)
Mincecraft also has the element of asyncronous Multiplayer implemented in a smart way.
Even if you log on a server, without other players populating the World
at this time, it still enables you to have some meaningful interaction with other players.
(Looking at new creations, building stuff to show later)

This is really important for small games starting with a
small playerbase.
Games like Onlineshooters or RTS simply offer no fun without
having several people logged on at the same time.
Causing the game to never reach the crittical mass of a playerbase. (those games die quickly, as noone is then logging
on anymore, since noone is logged on)


#4 Posing in WEB 2.0

Minecreaft is simply fun to watch (Youtube etc), seeing others playing the game, and showing their creations of funny moments.
There are not many games that are usable for this...

Fixel-level / objective game are
not much fun to showcase to others, as the gameplay repeats
for each player in more or less the same way.

The huge amount of fanmade play-videos and screenshots dragged most people into testing the game for
themself.
Its the perfect (viral) marketing tool.


#5 Indycharm

What Notch did was very smart. Up until now, his website
looks like an Indyproject website.
If he would have pimped up this site to look like a professional smoothed corporate marketingplatform,
people would have thought: oh, this must be some big company
behind this, using a well established massmarket-gameplay.
I will find them later in the web, lets look
at some other site.

Instead it brings this nice "homebrewed" Indycharm.
Instantly associating it with some probably cool inventive
gameplay and Artstyle.
"Lets try it, before I forget about this site later"

Also this: "Game sill in Alpha, but you can prepurchase it"
was a rather smart choice.
Making you think, the game will get better and better,
but cool, I can try it out already. Im "underground" and "In".
(like the other 1 Million people)


#6 Artstyle

Minecraft uses a very simplified pixelartstyle.
Along with other similar simplified art-concets (Tronlike Neonglow style,
grey-white abstact style, comic shader style, "handdrawn" style like Creonphysics
etc...)
it leaves the player with enough room to imagine things.
The more simple the style is, the more the imagination plays a role.

Also, by limiting yourself with the complexity of the style
its much easier to have coherent graphics, while
reducing the effort to create many assets in the same quality.