I installed L4D this Free Friday, played it a couple of times and didn't really get it nor really liked it. However there are many people that say this is one of best online games and I'm ready to give L4D a chance.

Campaign mode sucks even more then singleplayer, only thing different is the stupid AI is replaced by even more stupid real people.
Survival? Well, there are better flash based survival games than this, so I'm not going to waste any more words on it.
Only thing i somewhat liked is the versus mode, but I don't get it. It seems to me that humans must move as quickly as possible because of zombie's respawn time and use explosives on boomer horde. Zombies OTOH are much weaker and must use cooperation and map knowledge to their advantage. It looks like L4D doesn't require any twitch reflexes or aiming skill. It looks like a giant spam fest to me, like a team composed only of demomen on Dustbowl 3-2 in good ol' TF2.

The big question is this: What am I missing out? Is there something about the game that is so great and fun and I missed it?

Well, VS mode only kind of works if you have a gentlemans agreement not to just rush everything as well as 8 people who kind of know what they're doing. So pretty much never really.

Welcome to the L4D experience.

Survival is actually a boat load of fun if you play with cool ppl.

Like, one time we stacked all the firecans and stuff in one corner of the room, and it was all going to well until a fire that started outside spread inside to our little stockpile. There was this glorious "oh shi..." moment just before it all exploded and we all died.

Or the time some bright spark decided we should stand next to the cliff to lure the special infected off the side. Then the tank came and killed 2 people straight away.

Or how every single plan Fezovich comes up with fails miserably.

L4D is all about the people you paly with, paly with crap people then it will be crap, oh no you have to use cooperation and teamwork, oh noes devils duties.

Or, if you play with crackheads you can go to Dead Air finale and just kill and rape each other with no good reason.

It was consensual !

Yeah, I like these "We put all the things in one corner" moments. I always say "Someone will blow himself up" and thats what they do! My bad :3

Maybe you should go play a round with Mini Mcsplooge, but beware, hes crazy.

Dont play with randoms, join the HH !

Don't play with MJ or Mr.Bingles will die if you fail.
It's so much pressure, not to fail.
But luckily I've never failed and Mr. Bingles still lives!