It was an original and highly distinctive look for a spaceship. Looking at it after a moment, it starts to suggest the idea of flight, with those tubes flaring out like wings, and the saucer up front reminding the viewer of a UFO. The disc-like shape below is evocative of the headlight of a car, or even a figurehead, like on a sailing ship. Do you see the recurring theme? Objects designed to put people in motion, to suggest transportation, while being something entirely new.
If you like the look of the Enterprise, you have Matt Jefferies to thank. He served as art designer/production designer for TOS. He designed the ship, as well as the bridge, the ship's interiors, alien planet landscapes, and more. When you hear someone on the show refer to a ship's "Jefferies tubes," they're named for him.
What you have to remember when looking at TOS is, they had to maximize every dollar of the limited budget with which they worked. Often, that meant using and reusing sets, redressing them for alternate scenes, scavenging the studio lot for discarded items that could be used on a set. Jefferies made it work just enough to sustain the illusion of life on a starship or an alien world. Imagination did the rest.
Producer Robert Justman, in the book he co-authored with executive producer Herb Solow, Inside Star Trek, described Jefferies thus:
...Matt Jefferies was the most decent and devoted human being on the production team. He never lost his cool, never lost his temper. His eyes got watery and he would find it difficult to speak when an over-budget show forced me to take away half his construction money. And I'd demand the impossible, that he still provide us with believable sets for less money than it should cost. He'd gulp a bit and finally say, in a very throaty voice, "Well... let me see what I can do. I'll give it a try." So Matt would try harder, and he always came through for us.------------------------