The best example I can think of is Generation 1 Optimus Prime. To a philistine, it might seem cheap that he pretty much just looks like a truck cab that has sprouted arms, legs, and a head. That could be pretty lame...until you realize that the designer intends the windshields to evoke a chest's pectoral muscles, and the grill to evoke an abdomen. That is, in the case of a humanoid robot mode, the toy designer has looked at a vehicle and tried to figure out which parts of it looked the most human. (And many fan-favorite Autobots similarly deployed the front of a car to evoke a super-heroically-proportioned human chest.)
Other examples might include incorporating car doors as robot-wings (a choice that worked well enough in G1 that it was one of the few elements that was recycled in the otherwise gruesome Bayformer designs). Likewise, there is a degree of cleverness to recognize the angular shape of back windshields (or hood) as boot-like enough to serve as feet. Earlier transformer designs seem to take car to think about what vehicle components could translate into what robot components.
For negative examples, it is perhaps easiest to compare Optimus to the Go-bot called Stacks. Stacks makes almost no clever use of his truck parts at all in his robot configuration. He is literally a truck that can stand up and walk around. (Worse yet, he violates the headsculpt factor having only a windshield for a face.)
Even if the design is clever, it still needs balance. Throttlebots won no love for basically being the automotive equivalent of origami with robot heads stuck on top. Some additional robot features would have gone a long way on their design.
But toy collectors actually classify some other examples of poor design. For instance, they sometimes apply the term "brickformer" to a design in which the robot torso and sometimes face has been sculpted onto its vehicles undercarriage, so you pretty much just flip it up onto its trunk and pull out limbs to transform it. G1 Gears, Windcharger, and Brawn might all classify as this kind of Transformer, as would virtually all of the Stunticons. In this case, the robot is pretty much the bottom of the toy, and the vehicle is its back (modern RPM toys are an exaggerated version of a brickformer). There are also "shellformers" in which the alternate mode essentially cracks open and hangs off of the back of the toy like wings. You can see early iteration of "shellforming" with the G1 Dinobots, but the design really took off with Beast Wars and then Robots in Disguise. Shellforming, as opposed to brickforming, had the advantage of providing more articulation for the robot mode, bringing us all the way back to the first factor.
For a final negative example, the combination of features between modes can go too far, at least in my opinion. A major weakness of Beast Wars designs was that the already organic shapes of the beast mode often provided too simple correlations to robot mode. Optimus Primal, for instance, doesn't change all that much between his gorilla mode and robot mode. In many cases arms remain arms, legs remain legs, heads remain heads. The toy merely lets the user switch a claw back into a hand or foot, or flip a snout back to reveal a face.
I would suggest that this factor also helps explain why Generation 1 Transformers began to decline was that after the original animated film. Once Transformers started regularly turning into fantasy vehicles like futuristic cars and space ships, their vehicle parts didn’t really stand out in their robot modes. Cockpits, windshields, and wheels might stand out, but who could tell that Blurr’s shield was a hood, that Hot Rod’s wings were some kind of spoiler? Galvatron’s cannon/gun mode was completely unpredictable based on his robot mode, except perhaps for his forearm weapon. A well-designed Transformer has to be more than a robot with wheels screwed into its shoulders (sorry, Action Masters), and more than a tank with a head that pops out of its turret (sorry, minibot Warpath!).