We really should be using subthemes so that we can update base themes when needed.
Subtheming Quick and Dirty and Sub-themes, their structure and inheritance are good places to start.
Optional but good
404 Error page
There are modules such as logintoboggin out there to avoid these problems, but the way to create a custom 404 error is simple. Navigate to /admin/settings/error-reporting and point your 404 error page to a custom page.
Access Denied Page
Similar to the 404 error pages, Drupal has one of the most boring access denied pages. This information may not be detailed enough even for most people. They could think the site is down or something and leave.
A maintenance page, most commonly known as a "Coming Soon Page" can easily be done. There are times when your client wants to switch on maintenance mode. If so, all they need to do is navigate to admin/settings/site-maintenance and turn the site Offline. To theme this page just create the template page: maintenance-page.tpl.php
Special case forms
One of the forms which drupal provides is the "Are you sure you want to delete ___ ? Yes or No" It just looks horrible.. Other forms that get forgotten about are the built in "Advance Search" and "Reply to a Comment Form."
Be sure to check a few of the things that can get forgotten.
- Clean up CSS files
- Make sure there is a favicon. One can be created at http://favicon.cc/ if you don't have one handy.
- Make sure there is a thumbnail image for the theme list. Simply take a screenshot using the method of your choice, scale/crop it to 150x90 in the photo editing software of your choice, save it as "screenshot.png" and place it in your theme directory.
- Validate the results!
- Be sure you run major pages of your site through XHTML and CSS validation tests at http://validator.w3.org/
- Validate the design. Run the page through browsershots.