Wow, that post was vivid in detail and information.
Left 4 dead is easy if you want it too be. For me, I play on Advanced/Expert with FRIENDS for 2 reasons.
1. Other people try to rush through the stages without any type of planning, sure, it works on normal and easy but on the higher difficulty, it doesn't work too well.
2. It's EXTREMELY fun if you take your time and act like you would IF you WERE in your characters' place AKA not running into hordes without your teammates but looking around for useful equipment ALL THE TIME