I would add to the checklist that if you have EU or APAC countries to save the file as UTF-8 format or you will have funky characters. That kills me everytime.
If you're importing to an engagement program, its important to note they automatically become members and added to streams, I didn't know about this functionality and ran into some issues with it. I posted a question about it awhile ago here.
When importing make sure you understand each field and what it maps to. If there are fields whose values should remain unchanged then it's possible to set fields up this way in Admin > Field Management section (screenshot is below).
Also, if you have a list that contains a lot of special characters such as those commonly found on international distribution lists, make sure that you import in Unicode Text format; here is a link to another thread that covers this topic: Special Characters - é è ç à ï ä
Lastly, this point might be a little obvious but if it's a new email address then a new record will be created, which will grow the size of your database. If you are importing thousands of new people then your database will grow, and Marketo charges you based on the size of your database.