Error message that actually tell you what the problem is so you do not have to contact support.

Example: Everytime I tried to add an image to an inventory item, it would just say ERROR when I tried to save, with no further explanation. After spending 30-minutes on hold waiting for support, they told me I had to turn Online Ordering ON in order for the image to save.
It would have been nice to have an error message that said "Error: can not save image because online ordering is not turned on."