To escape, you have to survive going through a map full of zombies...
^^
WHAT SHE SAID LOL
but in all seriousness L4D2 has many different variations on the game
i.e campaign, pvp, survival and single player unlike the rest plus it has custom mods and maps too add to it kinda like counter strike which has its own world of custom maps mods
*which is how counter strike started BTW as a mod off half-life and became hugely popular as did day of defeat and team fortress and death match*
which BTW is why i fell hugely in love with counter strike bout 15 years ago
*yes that long lol*
because when you get tired of one variation ya can try another or try new maps or add new models and skins to make it custom to suit you
i.e zm, jailbreak,surf, gun game,super heroes, haunted mansions, ultimate gore hell they even have counter strike for kids
L4D2 is basically the same way and you can customize it and it will keep getting better
i have yet to get DayZ thou