every step of a brew is as important as the last one,sanatation,mash temperature,fermentation temp,whatever, there all important so there really isnt one thing you have to get right its the whole process..even the very last step of carbonating your bottles can ruin your beer..mind you thats not even the last step is it,storage temp and whatav ya..everything counts the same,you gotta get it all right
edit..i have just re read this post and i think i have misunderstood what you were asking..
