As you probably know by now, the demo is scheduled to come out in under two weeks. You may be disappointed to find out that of the four maps to be included, none will demonstrate the Warfare game mode. Mark indicated that a single Warfare map simply doesn't represent the game type properly, and therefore wouldn't do it justice.
The demo will officially be labeled as a beta, which means community feedback may indeed play a part in development. When asked whether Epic would be willing to delay UT3 if community feedback was not satisfactory it was noted with affirmation. However, it's unlikely that the community will have any major gripes. Epic has gone to tremendous lengths to ensure that UT3 is awesome, including scheduling two to three hours of play test every single day. The demo will primarily serve as a stress test and ensure compatibility over a wide range of systems.