How about we approach this from the affirmative first?
I suppose one thing that concept design/artwork involving Federation (and related) starships in the STAR TREK universe would be best advised to heed and follow would be to acknowledge many aspects of the canon design concepts seen in TOS, especially the original Starship Enterprise:
1: the Enterprise and her sisterships clearly had military capabilities, and formidable capabilities at that, as well as a military command structure. Having said that, they were not warships, nor did any character ever mention any existed. The Enterprise is repeatedly referred to/inferred as "the best ship in the fleet", and sentiment echoed in the very nature of many of the stories. (Earth and Starfleet don't just send a mediocre ship out on all these challenging missions, do they?) So, it is best to assume that Constitution-class starships (in the TOS era) represent the pinnacle in achievement in Federation/Earth design, shipbuilding ability, military capability and
sustainability for that era. That doesn't mean that every other type of Federation starship has to look weak or puny next to a Connie. That doesn't even mean there couldn't be alternative designs that may have limited quantities built for service. But the notion of fleets of multi-nacelled, armed-to-the-teeth Asskicker-class Federation warships doesn't fit in the TOS Universe.
2: The design of the TOS Enterprise is simple, and yet has subtle sophistication. I don't mind stating the obvious: there's an impressive elegance to it. There's a saucer section housing the bridge, crew habitat, other non-engineering facilities (sickbay, labs, weapons) and an impulse drive. There's a secondary hull housing a large flight deck, an apparent multi-level engineering section, a forward-pointing deflector dish, and presumably other high-energy/high-volatility technology that is best kept segregated from the crew habitat in the saucer. (One caveat: the saucer is repeatedly shown firing powerful weapons.) There is also the suggestion (never canonically confirmed or disproven) that the nacelles, while obviously serving as a housing for the warp drive's superconducting coils, may also serve as a logical place to store the bulk of the ship's fuel. These various hulls are connected through pylon-like necks. Note that the secondary hull serves as the central connection point, with all the pylons leading to it, not the saucer. That ought to say something about how starships in the TOS Universe are built.
3: The Enterprise's appearance in TOS also points to another design characteristic as a "hero ship": it looks "good" by looking distinct from multiple angles. This is accomplished by keeping the
various hulls dispersed enough (and oriented vertically and horizontally in relation to each other) so that the
hulls can be seen from
different angles. Despite this hull dispersal, the pylons connecting the various hulls are simple and straight so that they do not become too busy.
4: This hero ship visibility and simplicity extends to the smooth nature of the ship's hulls and pylons. They aren't busy with too many details, either.
I would also add a couple other subjective notions...
5: Warp nacelles should be balanced in pairs, commonly one pair, and if we assume that the nacelles house the bulk of the ship's fuel storage (presumably a substantial mass), some kind of center-of-gravity relationship with the impulse engines (as was
discussed in Shaw's deck plans thread over in the Trek Art forum).
6: If design concepts for other classes of starship are to blend in with the TOS Enterprise, then these other classes should also adapt the above design concepts. (Separate the engineering section from the crew habitat, make the design's hull arrangements "photogenic" like what was saw in TOS, etc.) As such, we should consider that some starships seen in subsequent adventures (the frieghters/cargo drones of TAS, the Reliant, etc.) might not fit in the TOS Universe.