It was my first Coachella. I was 18, still in high school, and had never attended a music festival before, so it totally blew my mind. The best acts of the weekend were The Whites Stripes and The Hives, who I'm surprised have not been back at the festival since then.
It was probably the last year that the polo fields actually felt sort of empty. it was very relaxed and crowds were never an issue. Obviously it's become much more crowded since, but 2004 brought in a lot of people for Radiohead, and I remember thinking that that felt a bit packed at times. People seem to regard 2003 as one of the weaker Coachellas, but not to me.