It's about two teams of four outwitting eachother - at least it is for me. If you're only playing offline or co-operative then you're really missing out on what L4D's all about - you're only playing bots, after all, and how dull is that?
In CS, you only really use 4/5 weapons and the same number of maps. There may be more - but they don't get used. Sometimes for a laugh, sure, an odd map is played, and league play tends to bring out the odd strange one to throw a spanner in the works, however you'll find that most cs players will spend the evening on probably a max of four different maps - the same old four, too.
Anyways, we'll probably just have to agree to disagree. I think we're coming at it from different angles, you seem to be after more of a single player experience, I'm more into online, competitive-type stuff.