From Good to Great

If you're brewing extract, don't worry about the water profile. Ideally, use distilled or RO since the extract already has a salt profile based on where it was made.

To echo others, it's all about fermentation. Pitch a proper amount of healthy yeast and control the temperature. After that's under control, add (some of) your extract late if you are doing partial boils to aid in both alpha acid solubilzation and wort color. Brewing Classic Styles has a nice explanation of how to determine how much extract to add early vs late for partial boils.

 

You make wort, yeast makes beer. Pitch enough yeast and oxygenate your wort well, and your off to a good start on improving your beers. Oh, and fermentation temp as mentioned.

Recipe formulation will take a bit of trial and error, but stick to simple recipes to learn what you're doing.

 

See my response in this thread: http://beeradvocate.com/community/threads/extract-vs-all-grain.83094/#post-1204948
I won't say that the changes in brewing practices referenced in there took my beer from good to great, but they did take homebrewed beer in my house from being a novelty to being something that I wanted to drink and serve to guests and periodically send off to competitions and, less periodically, win prizes.

Another important part of the journey is learning ingredients and developing a sense for the ones you want and the amounts you need. This won't come overnight.

Greatness remains elusive to me; I suspect it comes from applying advice from well-trained palated to the fine-tuning of recipe and process, and rebrewing several times, gradually incoroporating subtle changes until the beer moves from good to great.