Valve has released statistics on the achievements in the PC version of Left 4 Dead, which shows that the game is very hard.
For example, only 2.1% of players have even completed a single campaign on the Expert difficulty. Only 1.7% of people have been able to complete a campaign without using a first aid kit on themselves. Scary!
Check out the full list here and then let us know which of the hard achievements you've managed to snag.