Truax was advocating "design for minimum cost". As this paper here (starting on page 140) explains, this philosophy of design would probably include a number of changes to how spacecraft were designed.
I think this sentence is the key:
The fundamental premise that the DFMC concept rests upon is that, by
using a clean-sheet design approach, a space launch vehicle can be
optimized for minimum cost, instead of being designed like current
launch vehicles, which are optimized for maximum performance and
From reading through the section titled "Cultural Changes to Get a Space Truck", I think that the main point of the term "space truck" is to emphasize that it doesn't really matter how pretty or elegant the final design is, it only matters how much it costs to get the payload into orbit.
For non-engineers this sounds completely obvious, but it goes completely against the grain for many engineers. "Elegant" design (where the exact criteria for what is elegant varies from discipline to discipline) is considered particularly beautiful and is often striven for at the expense of other concerns. (Yes, I have been guilty of this) Mostly, this is a matter of culture (simple, robust design is rarely stressed, either in training or practice) and egotism (engineers like to brag about the beauty of their designs).