I like to think of Technic approach to aesthetics similar to the one somebody once described about high-end sportscars: their good looks are always a side-effect of technical solutions. It's form following function, and their aesthetical roots are in the engineers' laboratories and wind tunnels, rather than clean sheets of designers' paper.
Most original Lego Technic sets follow this principle. Except for a few partial exceptions (such as the 8051 bike reviewed here on Legoism), those sets contain very little dedicated decorative parts, if at all. But despite that, their bodyworks, fuselages and similar structures usually look great. Where is the fine balance, or in other words, what is the guideline for making useful technical things look good as a side-effect?
As far as I can judge, there are few points worth considering during planning and building. They are somewhat vague as they vary among models and purposes, but in general, most good-looking models obey them.
▪ Use reinforcements as decorative elements or body contours.
Check out the 8865 Test Car. While its side beams between the front and rear wheels clearly serve as reinforcements and support for the seat plates, they also nicely define the bottom side contours of the bodywork. The same counts for almost all outer beams on the car: while each increases the structural strength of the bodywork and the underlying chassis, they are all cleverly positioned to define the outer edges. Many models follow this rule strongly, but on 8865 it is particularly evident.
▪ Go minimal with plates; try to keep as much inner mechanics and controls easily visible and accessible as possible.
One of the advantages of aforementioned reinforcement design is its transparency. Seeing "the things happening" inside is a part of what makes Technic what it is. Although attaching the plates to the beamy skeleton underneath may provide more pronounced or strictly realistic bodywork, that is rarely desired in Technic. Though, even some early original Technic models do not follow this rule ― but neither are they among the Technic's nicest, if you ask me.
▪ Small details are welcome, particularly if they emphasise the various components' functions.
If not hindering mechanical functions or hiding controls, occasional small details can nicely enhance the model. For example, mounting a small rotating fan on the car engine will add to looks and realism, even if it doesn't really move any air. Just as little wing mirrors could be a nice touch if it's obvious that the car is otherwise missing them. But in all these cases, decorations should strive to make the model look more realistic, rather than just put an unnecessary gizmo somewhere for the sake of making things more complex. Example? Look at the recently reviewed 8053 Mobile Crane ― it has several lights on the front of the cabin. The model would be just as functional without them, but the front mask could look somewhat bland. Well-positioned and subtly fitting into the overall design, they are a perfect little touch. But the concentration of such details is here wisely judged, too: adding the front air intakes and wipers would just make the model look to fiddly.
▪ Try to follow the structure of the models' real-world counterparts as closely as possible.
There will always be variants and controversies, but in general, most machines in the real world have their proportions and structure following some well-established guidelines. Let's again use cars as typical examples; there is a certain ratio between the chassis length, width, engine size, wheel size, cabin volume and many other parameters that almost all cars follow within certain extents. If you're building one, plan staying within those limits ― otherwise the disproportional structure might look inelegant, even if there is actually a good sane reason for doing so. You might often find yourself having to choose between mechanical functionality and realistic look. Check out the majestic 8880 ― every major component's size roughly matches the general car dimensions, and it's approximately where one would expect it in a real supercar.
▪ Don't make it look like a carnival in Rio.
This one comes the last, as it's the most obvious and ― due to the possible lack of choice ― often difficult to follow consistently. Namely, technical functions (which should be a clear №.1 in Lego Technic) will be easiest to distinguish and understand if the viewer is not distracted by twenty different colours around. Basic grey shades in which most Technic parts are made work nicely, and often adding one extra color can make it more intriguing. But if you can choose, resist making a colour blindness tester, like I did: