I want to create a site that lets forum owners come and download forum themes. Similar to http://www.awesomestyles.com. Would Drupal be good for this?
Yes, Drupal is suitable. You'll have to use modules to manage downloading of files and permissions (for instance if you want only registered users to be able to download some files while other files are available to all)
Use Project module for theme management. This module is used on Drupal.org for themes and modules. Then, check out Views and CCK (Fields API in Drupal 7). With CCK you'll be able to create fields of all sorts – text field for Theme's author, or select list for Version compatibility, for instance. Finally, use Views to easily create views of SQL-queries of any complexity. Say, you'll want themes to be displayed in grid, 3-4 in a row, with large thumbs, license information, and some features aside. Or, you'd want to sort themes by some parameter, like Clean/Blog/Corporate/etc. Apart from these three, there are over 7K other modules. It's too early to use D7, use Drupal 6 by now. Make sure you view Showcases and read case studies where people explain how they built their websites using what.