Of the four cardinal virtues -- perhaps of all virtues -- temperance is the most difficult to pin down. Most virtues have their puzzles; virtue touches on so many things that's bound to happen. But few virtues create as many puzzles for analysis as temperance does. Discussing justice, for instance, is immensely easier, because justice is the virtue that deals most closely with obligations. You can get rigorous distinctions, strict requirements, endless ranks of analyzable and often analyzed duties, offices, and laws. These things are available just by the nature of justice. Other virtues are not usually so analysis-friendly, but they usually provide multiple handles for analyzing how they work. Temperance is a bit analysis-resistant. Even Aquinas, who can analyze how virtues work like no one else, often has to 'cheat' with temperance by looking at how it interacts with the hard & fast lines of justice.
Temperance and its satellite virtues are fuzzy virtues by nature. Temperance deals with balancing; it deals with questions of what you are communicating to other people by your actions, and thus with symbolism; it deals with what people generally would admire and respect; it deals with questions of appropriateness. What it involves is very sensitive to circumstances that can vary considerably from person to person. You should be temperate with respect to alcohol; for some people that will be total abstinence, and for others a small amount ever once in a while, and for others could be consistent with an amount that would make the second group inveterate drunkards and radically harm the first group. If someone you don't know at all comes up to you and asks how much they should drink, you won't honestly be able to give them more than a vague answer. Temperance and its satellite virtues are mostly like that; at some point, the best you can say is, "Yeah, you just have to use your best judgment."
People constantly are trying to ignore this, and it gets them into trouble. We talk about prudes and puritans; to the extent that these indicate real negative qualities rather than just being rhetorically charged words, they generally mean people who are trying to take matters of temperance, that require finding a good balance for the person and situation, and turn them into matters of some strict, one-size-fits-all rule. It's a good way to become intemperate. The boor, the stick-in-the-mud, the wet-blanket, very often develops from someone who begins treating jokes and playfulness as if they were things that could be done on strict schedule and inventory of topics, and by strict guidelines. The opposite is often explainable the same way. Why are so many people tempted to lax views in matters of food, drink, and sex? Because 'anything goes' or 'anything goes that doesn't harm someone else' or 'anything consensual goes' are rules much easier to follow than the rule that you need to find a balance, appropriate to a rational and reasonable person, that takes all of your circumstances into consideration and treats more important things as more important.
There are indeed definite things that can be said about temperance. It is impossible to be virtuous without it, as has been argued since Plato's Gorgias at least. If we are taking the term generally, it is rational moderation with regard to desires and pleasures; if as a specific virtue, with regard to the most physical desires and pleasures. It is the virtue to which 'moral beauty' is most attributable because it deals with balance, appropriateness, and proportion; on similar grounds, one of its concerns is making one's actions communicate the importance of reason and virtue. It requires making a distinction between what you actually need as a rational person, and acting accordingly; it requires treating higher priority things as higher priority. It concerns matters of shame and honor, and makes use of both. It does have some hard, fast lines: some things are just inconsistent with rational moderation, or cause insuperable problems for trying to be so, and some things are matters for temperance but also touch on other virtues that restrict the options available, as (for instance) sexual matters need to be approached not only temperately but also with justice to all who are or could be involved.
One could imagine a number of possible lines of inquiry that would likely turn out to be fruitful for improving our understanding of temperance. It deals with moral beauty, so accounts of moral taste might shed some light on it. It deals with shame and honor, so philosophical examinations of these and related things like etiquette might be fruitful. We've done some more exploration of the idea that moral actions are communications, in some strands of phenomenology. There is much to explore. But at the end of the day there is only so far that we can ever get. Temperance is a kind of acquired genius for beautiful life. As there is no one way for a painter to make a beautiful painting from given materials and details, there is no one way to build a beautiful life. You can perfectly well talk about better and worse ways; but even more than with other virtues, there are going to be many situations in which I would be intemperate doing what you can do temperately, and vice versa, and there will also be many cases where it might be fine for us both but more fine for one of us. As the saying goes, there's a kind of art to it.