After playing Left 4 Dead 2 extensively, I went back to the original to beat my missing Blood Havest campaign, and boy, the game is a total cakewalk compared to the sequel. I cleared Blood Harvest in only 1:28 hrs and only re-started 7 times. I also revisited the first two other campaigns and I cruised through them relatively easily this time around.
The only thing that is harder in the original are Tanks, but otherwise, without jockeys (I hate those guys), without fatigue, without uncommon infected, and without gaunlet crescendos - the game is substantially easier.
Anyway, I've beaten both Left 4 Dead games completely now and got the respective What Are You Trying To Prove? achievements, so yes, mission complete!