As a student, I have spent a lot of time experimenting with pasta and tomato sauce. These are my entirely subjective results.

Good Tomatoes

When asking people what makes a good tomato sauce, probably the most common answer you will get is "good tomatoes". It is a conceptually nice answer for several reasons. Tomatoes are universally seen as healthy - we like the belief that fresh and healthy ingredients make good food, and "good tomatoes" should be the only logical answer because tomatoes are the main ingredient. Furthermore, it is an answer that makes you comfortable since if your tomato sauce doesn't taste as good as the one on vacation in Italy, there's nothing you can do because they just have access to superior ingredients.

Then what actually makes a good tomato sauce?

What actually is a good tomato sauce is already a quite difficult thing to find out. There's an interesting talk by Malcolm Gladwell at TED where he speaks about clustering of people's preferences. There seems to be a large group of people who like their spaghetti sauce extra chunky, and several other clusters.

However, I've come to the conclusion that the aforementioned "good tomatoes" actually don't play a role at all for a good tomato sauce. Making tomato sauce from fresh tomatoes is a mess, and there are a lot of things that can go wrong. You need very ripe tomatoes, you need to peel them, get rid of the kernels... and worst of all, the result is often indistinguishable from the sauce you make from a can of preserved, diced tomatoes. Also, I haven't been able to distinguish organic tomatoes from regular ones - this seems to be a primarily ethical choice.

What actually does seem to matter is the content of salt and fat, which is in line with research findings. People like salty and fatty foods (no shit, Sherlock!), so if you go for taste and not health, you may want to add a good gulp of olive oil. Probably after cooking, in order to preserve the taste. Fats are also natural flavor enhancers. By stirring it in, you can create an emulsion of little olive oil droplets in your tomato sauce, enhancing the natural tomato taste.

Speaking of taste... a good Italian style tomato sauce needs basil, the more the better. Fresh basil, that is - the dried version tastes completely different.

Variation

If you eat the same tomato sauce for a while, it becomes bland. Luckily, there are lots of ways to experiment, since you can add oregano or leave it out, add some chopped onions or mushrooms, red wine, powdered paprika, tomato concentrate or garlic. Of course, you can also experiment with different cheeses on top. The result is sometimes better, sometimes worse than what you had before, but always slightly different.

tl;dr: Tomato sauce is awesome, add lots of olive oil and basil :)