I first noticed ACS when a former colleague pointed me to the hilarious recruiting page of arsdigita: http://web.archive.org/web/20000510060828/www.arsdigita.com/pages/jobs/recruiting (archived without the Ferrari photo unfortunately). From there I started reading philipps books, the bboards etc. - a typical route propably. I was working with java and php on developing web systems then, which tended to reinvent common concepts again and again, so I was easy to convince I guess.
The two concepts that drew me most to (Open)ACS, deserve to be emphasized again, although they have been mentioned already:
- Two-Tier Architecture - http://philip.greenspun.com/panda/databases-interfacing
- Reusable code