It gets tougher as time passes by. That means by minute 40, you are playing at hardcore difficulty. This makes for an experience that is good for learning the mechanics as once you can not advance due to the difficulty you will die which means the game has to be repeated which help learn the ins and outs more and more each time it is done.

The levels are generated very randomly making for a unique experience every time the game is played which in turn makes for a ton of replayability.

Imagine a small set of DOTA or LOL. These 10 characters can be tankers, DPS, supporters, rangers, carriers, etc depending on your skill and level of use. And they're hard to unlock which makes the challenging to get them even more.

No need to compete as the multiplayer is all cooperative where each player has to help the other survive the harsh world which is a fun setting for friends as it leaves out any competitiveness.