Nope not played Left 4 Dead yet, hope to soon though .
Lol if you're easily frustrated thats quite likely .
The normal enemies on Extreme are deadly, the bosses are very powerful, plus those data posts don't give you thermal energy any more so you have to play fast or you'll freeze to death. Its just all round a very difficult mode to play.
Yeah I took a quick look at the games achievements, for the most part its the same as the regular version, still need to complete the game 4 times , plus it has reach level 100 in online, the regular is only 50 so that will likely take a while.