I think this can scale nicely as fits into Venkatesh Rao's "warren" concept: http://www.ribbonfarm.com/2010/10/27/warrens-plazas-and-the-...

Two things:

1. Throw some HTML5 gradients at this baby - they're not that hard - cheap way to look web 2.0

2. Do create anonymous users, but architect it so that all user profile storage is browser local - this keeps your costs way down - and gives users an incentive to register as they can then use it in other browsers.