If you look at AI-written text, one major problem with it is the overuse of sentence fragments, very short sentences [1] and, more generally, a staccato style of writing rather than having the discourse flow. This can make the writing feel hard to follow. It's sort of the opposite problem of run-on sentences. Overuse of metaphors is also something that is counterproductive when you want to be understood, and LLMs make them a habit.

You can combat that to some extent by giving the model a style guide to curb the worst excesses, but some are likely to still slip through.

It doesn't bother me much personally, but for plenty of people it's a nails-on-chalkboard thing.

[1] For example, the problem with the em-dashes is not the em-dashes, but that it's just another case of getting hit with punctuation every few words.