In no particular order....
10. Their usage policies are vague and arbitrarily enforced. Campaign materials for both candidates were featured on the home page, but technically anything featuring the likeness of a celebrity is in violation.
9. Their web interface is poorly designed, and requires far too many page refreshes for the world of web 2.0 to deal with.
8. They don't let you see records of sales for periods less than 3 months.
7. If you make less than $25 on your account, they never pay you
6. There's no easy way to change the design on all of your products at once; you have to do it for each individually. Want to resize that image on the 20 different types of shirts you sell? Better block off an hour.
5. The ability to design shirts is ridiculously constrained. You can upload small images that will be displayed in small areas on the front and back only.
4. It's really expensive. $12 for a plain white t-shirt. You only get a percentage of your mark-up.
3. Their customer service is frustrating. When one of your images gets blocked, you receive a completely un-informative form email informing you of the block without telling you why. It's up to you to guess and then reply with reasons they should turn your account back on.
2. They require your social security number "for tax purposes" when it's likely they'll never send you a check.
1. DesignAShirt.com and CustomInk.com are easier to remember. It's weak, but I needed a 10th.