Upgrade & Billing Screen Design Walkthrough (Basecamp)

I think the biggest selling point on their final design is the simple call to action Get More Done. I have used basecamp in a limited fashion, but it seems that what really makes their final design stand out is the benefit that effectively state in their call to action.

They had several good designs that helped people understand what they would be getting by upgrading, but that call to action is, my guess, what will most likely help users to upgrade.