OpenACS is a toolkit for online communities. This is a pretty powerful concept. If you think that most of the successful websites are community-oriented, you'll begin to see the power that OpenACS brings to you.
Most people get too focused on how their website is going to look, rather than thinking what it's going to provide for the users. Looks are important, but if you don't have content and services well organized, you can have as much looks as you want. Yahoo.com is pretty much all text-based, but it's fast and provides lots of content to users, or "magnet content" as Philip Greenspun calls.
OpenACS has over 40 modules to facilitate collaboration among users. You need to think which of these modules you can use in your web service and what's the best way to do it. A complete list of OpenACS modules is at http://www.arsdigita.com/pages/toolkit. After you have that planned and defined, you can start worrying about look and feel. You should also read the Webmasters Guide, which is at /doc/webmasters.html.
Refer to Philip and Alex's Guide to Web Publishing for more info on how to build great web services.