Fermentation temp control is the most important step after sanitation... but it is also a very difficult one to manage. Here in CA a lot of brewers can't get their wort under 75 degrees without expensive equipment... and maintaining proper fermentation temps without a dedicated freezer/fridge is basically impossible 9 months out of the year. I always tell people as soon as they get into the hobby to look into a used fridge or chest freezer... I just can't abide by hot ester bombs that get made when folks pitch at 78 and ferment in the low 80's.
That said, most of the other costly equipment is all about convenience, not better beer. I don't need my mill, pump, or fancy chiller, but they make my brewday easier. That said, I would never advise a new brewer to shell out the cash for something like that, but I always tell people they should manage fermentation temperature and that my preferred method is with a dedicated "fermentation freezer."