I'm not convinced that mashing with base grains vs. steeping alone would have such a huge impact on the beer. It can be argued that there are some more complex carbs that are not broken down into fermentables when steeped (that would otherwise be when mashed where more enzymes are available for conversion) that could attribute to more sweetness/flavor/mouth feel - this could be partially concluded if the FG of the extract brew were higher than the AG brew. But, there are many other factors that would affect that as well (quality of extract). Another possible factor could be the use of extract vs. AG somehow producing a more pleasing beer - type of grain used, mash temp, or fermentation parameters as you alluded to.
Make it so!
Secondary: Doppelbock, DIPA
Barrel Aging: Flanders Red
Bottled: Ginger Mead, Scottish 80-/
Kegged: Belgian Golden Strong