We need to use language when outlining a theory, but what definitions should we use? Hume notes that he's aware of the potential for misunderstanding and has been careful about the words that he's used in the Enquiry.
Hume argues that it's hard to draw exact boundaries between virtues and talents (or vices and defects) and to precisely define these terms, but he follows this up with a reassuring point: this is just a matter of words. Phew.
He adds that, on the whole, there seem to be virtues of many different kinds, but calling a person "virtuous" is to chiefly praise their social qualities. Someone may be good-natured and honest, but if they're totally lacking in courage, industry, self-control, and dignity, they're hardly models of virtue.
Ultimately, one reason why languages struggle to draw a line between virtues and talents—and vices and defects—is that we don't make much of a distinction between them in our minds. It's more a matter of definitions than how we naturally think.
One factor that makes a quality valued is its rarity. Good-naturedness and honesty are seen as requirements, so possessing them is no biggie. It's when someone defies these codes that we're like, "whoa there!" Intellectual qualities tend to be seen as rarer than qualities of the heart, and that's why people are less likely to congratulate themselves in this area: they don't want to be seen as big-headed.
In general, Hume feels that virtues of various kinds can come about from similar sentiments. Sure, the praise given to industry, self-control, and cautiousness may be different than the praise given to other kinds of virtues, but that doesn't make them a different species. The ancient moralists, for instance, didn't make a material division between different species of attributes/defects. These were either virtues or vices—period.
Modern philosophers have often taken a different outlook, with philosophy having become closely linked with theology: a school that, in Hume's view, has warped every branch of knowledge and language. Nope, he's not a fan.
Hume wraps us by stating that a vice, a blemish, or a crime are different expressions but are cut from the same cloth. Likewise, the duties we owe to ourselves and to society probably work from the same principles and attract similar praise. So let's not obsess over this stuff, m'kay?