At the time ‘world at war’ was the other popular zombie game and it’s also pretty simple by today’s standards.
I like the lack of progression, it’s nice to know everyone starts the same with each game. No special meta build just to enjoy the game.
I think the game back4blood seriously shot themselves in the foot by lacking the co-op verses mode l4d had. It’s the biggest complaint I hear against it, that and the lack of workshop support to make your own campaigns and content.