1. At the very beginning of work don’t forget about original prefix for tables in DB.
2. Check all site pages in IE 7
3. Make sure, that admin with limited rights is developed for client, ant that this admin has enough rights for content editing.

4. Make sure that visual editor is implemented and client admin has rights for using it.
5. Check all error pages, you can use Custom Error module for it.
6. After deployment make sure that poormanscron module is installed or check that launch of cron.php from regular procedures of server is adjusted correctly.
7. Make sure that anonymous has access to site content, check all functional as anonymous and check it as user of every other role also.

If you have something to add - feel free to make this list longer!