When I first started I approached brewing like I was making a cake. I got recipes online or at my LHBS, followed the instructions, and ended up with beer I was proud of, but wasn't all that great.
Soon I started formulating my own recipes. They were a generally a muddled mess of ingredients. I didn't fully understand how the flavors would interact. I made mediocre beer.
The first big shift in my thinking happened when I started looking into water chemistry. I think the importance of water chemistry is overestimated by most amateur brewers. Getting your mash pH in line is crucial, and is related to water chemistry, but trying to imitate "historic" brewing waters is a terrible idea, from my experience. The worst non-infected beer I've ever made was a pale ale with Burton-on-Trent water.
The next shift in my brewing philosophy was to pare my recipes down to make them as simple as possible. Plain old 2-row has a lot of flavor. I started thinking about the types of flavors I wanted in the beer, and adding ingredients with purpose. My brews turned out much better.
The latest shift in my brewing philosophy came when I realized how important proper yeast health is to good fermentation. More on that later.