Verbiage is verbiage, it doesn't necessarily modify between lower and upper levels of usage besides finding more discrete, however uncommon words to speak in a more specific capacity than general language. Words that people may rarely hear, or never, something like "pulchritudinous" (which is a bit obscure imo) or "circumambulate" (which wholly explains itself and I think is a quite pretty word). But I would consider the delineation between more basic or advanced usage of language, of any language, not just English, to be distinguished more grammatically or syntactically than anything else. The kind of thing that someone would say they "struggle to read" because it has too many pauses, grammatical flourishes that they are unfamiliar with, it isn't "plain" writing in any sense of the word. (And since you don't explicitly "perceive" grammatic annotation in speech, the only way someone would confuse something when speaking to someone is according to a loftier employment of terminology, something perhaps related to a technical field, like the meme of a doctor speaking way over their patient's capacity for understanding.)
In an example of the difference between basic English, which can be seen in 99% of HLTV posts and needs no illustration, and more advanced English, I would highlight something such as the opening paragraph of Melville's "Moby-Dick," which I don't consider at-all difficult to understand, but would certainly turn some people off who are only used to the expediently described Twitter/text-message syntax of informational delivery. Consider:
"Call me Ishmael. Some years ago—never mind how long precisely—having little or no money in my purse, and nothing particular to interest me on shore, I thought I would sail about a little and see the watery part of the world. It is a way I have of driving off the spleen and regulating the circulation. Whenever I find myself growing grim about the mouth; whenever it is a damp, drizzly November in my soul; whenever I find myself involuntarily pausing before coffin warehouses, and bringing up the rear of every funeral I meet; and especially whenever my hypos get such an upper hand of me, that it requires a strong moral principle to prevent me from deliberately stepping into the street, and methodically knocking people’s hats off—then, I account it high time to get to sea as soon as I can. This is my substitute for pistol and ball. With a philosophical flourish Cato throws himself upon his sword; I quietly take to the ship. There is nothing surprising in this. If they but knew it, almost all men in their degree, some time or other, cherish very nearly the same feelings towards the ocean with me."
Melville was imo one of the few masters of English, and his English is certainly very advanced throughout his novel even for native speakers to have trouble understanding it, and not merely as a caveat of the era or "age" of his dialect (when they still used words that have fallen from contemporary usage, like "ere"). Moby-Dick in fact has one of the highest counts of unique words among all English novels, average a new unique word usage, by numeric count over 16,000, averaging somewhere around one per every single paragraph (rough memory speaking). Attempt to write a book that spans 660 pages and use a new, unique word in every paragraph, I'm certain you will feel yourself challenged short of even the halfway mark.