• It goes on too long, and player listening comprehension takes a nosedive.
• Because it's spoken aloud exposition, it should obey different rules than ordinary running text (which is intended to be read).
Example: I love this bit from Clark Ashton Smith's "The Ice-Demon," which I'm altering a bit so it's second person and present tense like D&D readalouds. My changes are in italics, but really, all I changed was tense and person.
The entrance is like a fanged maw. Within, the floor slopes downward at a slippery angle for more than a hundred feet. The chamber swims with a cold and glaucous translucency that filters through the dome-like roof. At the lower end, in the striated wall, you see the embedded shapes of a number of men, among which you distinguish easily the tall, blue-clad corpse of King Haalor and the dark, bowed mummy of Ommum-Vog. Behind these, the shapes of others, lifting their serried spears eternally, and receding downward in stiff ranks through unfathomable depths, are faintly discernible.
Good stuff! But if a DM reads that aloud at most tables, it falls flat. Too few of us--designers and editors alike--actually read our readaloud text out loud. It reads good, but it's hard for players to really take in when read aloud. Reading comprehension and listening comprehension are very different things.
Imagine you're a player at that table. Even if you're paying rapt attention, you have D&D-style questions when it's all done. You want specifics: How many men? How far up the slope to the entrance? How far into the back can we see before it becomes "unfathomable?"
And really, at a typical D&D table with dozens of distractions, the above description asks too much of the players' listening comprehension. If players were reading that text or looking at a picture, no problem. Message received. But when they're listening to someone read it aloud...comprehension plummets.
If you accept my premise that existing readaloud text is a weak point in most adventures, consider my solution: Write your readaloud text like dramatic dialogue. Use the oral storytelling tradition, not the written literature tradition. That demands different word choices, different sentence structure, different everything. But the payoff for the DM is that you'll keep the sinister mood of Clark Ashton Smith without repeating yourself a dozen times.