Package delivery to your resort, if you use it, would be another missed perk.
The ability to charge items back to your room, if you use it, would be another.
I'm not really in a position to compare, because if I stay off property, I stay home and do a day trip to the parks, so for me a WDW vacation definitely means staying on property. I like having the option of NOT driving back and forth to the parks, especially at the end of the day when it seems like everyone is trying to leave the parking lot at the same time. Pretty much the only place I drive to is DTD, because it takes FOREVER to get a DTD bus. I like being able to charge items to the room so I minimize the number of people handling my credit card (in fact, I do online check in for this reason as well) and I like being able to send large or fragile purchases back to the resort instead of trying to juggle them on rides or transportation (even if you drive, you have to be able to handle them on the parking lot trams). So my answer would be that I would stay in a moderate before I would stay in an off property resort - but each family has to make that decision on their own.
You can have a great trip staying off property. You just need to decide which amenities are worth giving up for the price, and what amenities (such as space) you might gain.