The ease of use when setting up new products or creating an account for existing customers - everything you need to know is right there in front of you! I don't like how hard it can be sometimes if one part isn't working properly (e.g., adding product photos).
This has not been much of an issue as our customer base hasn't expanded very far yet so we haven't had any issues with this. We are using 4Psite primarily for order management but have also used their reporting functions which were easy enough once set up correctly.