One of the lessons I have learned is that strings matter a lot. This was taught to me in college when I took a couple semesters of classical guitar by a great teacher, but I have also heard this from professional musicians as well. I suppose another reason I am particular about strings is from when I learned how to play alto saxophone. My private tutor taught me the difference between reeds, and how the quality affected the sound.
The brands I was advised to buy and chose to pick are D'Addario, Ernie Ball, and DR Strings.