Using the Zen theme from the folks over at Palantir.net simply put, is great! I've used a lot of base themes now and the biggest thing I despise is a base theme that makes me feel like I've lost touch with the mark-up. Zen is truly a bare bones theme with a lot of pre-configured features including HTML5, HTML5 shiv, Responsive Design, Sass/Compass, CSS, Normalize CSS, Respond.js, IE conditional classes, Drush support and Accessibility. This saves me at least two hours getting Drupal ready for theming.
Drupal Multi-Site set up can be difficult if you don't have good direction. When I started my journey of setting up a multi-site I wasn't sure where to start so obvisously I started at drupal.org to try and get a good idea. Well like most thing at drupal.org some of the info can be a little cryptic if you haven't been doing drupal since Drupal 5. The next step is to sift through good ole' Google. It's safe to say there's a ton of information out there. The general gist is that if you want to have one actual drupal installation and share the code base amongst many sites that depend on the same modules this is the path for you.