I've noticed when people get to arguing about how a particular factor affects beer they always seem to go on as if it is of prime importance. I'm not always convinced. For example, home brewers seem to spend an inordinate amount of time obsessing about liquor treatment, when from what I can see ion balance has a relatively minor effect.
Principles of Brewing Science he classifies a number of factors as being of primary, secondary or tertiary importance. Mastering primary effects he says is crucial for every brewer, whereas the secondary and tertiary effects are fine tuning.
Here's his ranking:
Relevance in brewing:
Hops and hop chemistry
Water chemistry and water treatment
It's interesting to see where he places things, but when I try and slot things in he doesn't list I keep thinking "it depends", with things such as the beer style and if you do get it wrong how badly being variables. Also as he's talking about brewing other important points such as carbonation and the use of sparklers isn't listed. I shall ponder some more.