Every Module that I have downloaded so far from the official Drupal.org website has had major issues. The majority of what I have seen has been Browser compatibility issues. If the themes will be listed on the drupal site then shouldn't they be pass some set of standards?
Steve Yelvington has a good article here: http://www.yelvington.com/why-do-drupal-themes-suck
Conclusion: One must take the time to design personalized themes and not rely upon the official repository.
In case you are having trouble with installing Drupal for use with multiple websites this should help. Drupal's install.txt file that comes with the basic install package has a pretty good short explanation. This may not be enough especially if you are on a shared host. Some of the configurations may not be possible due to your shared host's default settings.
There are some pretty good tutorials online.