For me, it's the people...
By and large, the facilities aren't head and shoulders above the comparably priced competition, but IME I've always gotten consistent, courteous and professional treatment.
I'm not as well-traveled as most FT'ers, but, as a pre-requisite to buying into MVCI, I stayed at a wide variety of quality levels (from Cat 1 to Cat 6) of Marriott properties and the experience is what sealed the deal. The top-drawer facilities at their timeshare properties are just icing on the cake.
Just as I've found by trying different cruise lines, one usually finds a vendor which "fits" well with one's needs and desires. In hotels, that "one" for us, at this time, is Marriott.
Pat