Formal, Wordy, Descriptive, Repetitious
Works of philosophy aren't the most fun, frothy texts in the world, and their language is usually on the serious side (like in an academic essay). The Enquiry is no exception, and the fact that it was written in the 1700s means that Hume sometimes uses words, spellings, and references that aren't as fresh as they were back in the day ("connexions" and "knave," anyone?). Still, modern academic/philosophical works have their own brand of highfalutin' language, so it's not like Hume's writing is any more difficult than what we're used to.
Some of the sentences in the Enquiry are lengthy and use terms that you may not be familiar with. Take a look at the first sentence:
DISPUTES with men, pertinaciously obstinate in their principles, are, of all others, the most irksome; except, perhaps, those with persons, entirely disingenuous, who really do not believe the opinions they defend, but engage in the controversy, from affectation, from a spirit of opposition, or from a desire of showing wit and ingenuity, superior to the rest of mankind. (I.1)
That's a mouthful, right? What he's saying, though, is that stubborn people can be annoying but they aren't as bad as folks who don't even believe what they're saying and are just trying to show off/get a rise (one word: trolls).
Despite his wordy, formal language, Hume helps us out by repeating and emphasizing certain words. Check out this quote:
Who sees not […] that whatever is produced or improved by a man's art or industry ought, for ever, to be secured to him, in order to give encouragement to such useful habits and accomplishments? That the property ought also to descend to children and relations, for the same useful purpose? That it may be alienated by consent, in order to beget that commerce and intercourse, which is so beneficial to human society? (III.II.7)
One thing that we need to remember here is that Hume didn't want to just cater for fellow academic types—he wanted his work to engage with a wider readership. He therefore keeps returning to his main points and buzzwords, as we can see from the references to "useful" and "beneficial" (plus the repetitive wording and structure) in the quote above. We often think of repetition as a bad thing, but Hume uses it to get his argument across and keep it in our minds.
Hume's style may seem daunting at first, but once you get used to it, it's really not so tough. Even if you don't recognize every word or reference, you can follow Hume's train of thought and understand his overall argument. Plus, we have to cut Hume some slack—we can't expect a centuries-old philosophical work to be a complete breeze.