Sending off First Edits to an author is always a nerve-racking experience. First Edits by their very nature are a tricky balancing act between suggesting possible changes which might improve a book and insisting on certain changes which are vital to the credibility and coherence of the work as a whole.
These changes can be straightforward: continuity problems when characters forget what they have done in the past, change their eye colour mid-novel, or even change their names half way through.
They can be dialogue-based. Every character speaks with the same voice, uses the same favourite slang word or expression, or the same grammatical constructions.
Often they are to do with the pace of the novel--no hooks at the end of chapters so no tension, too much action and no dialogue so no reader identification with the protagonists.
Sometimes, a writer has a penchant for overusing adjectives or adverbs. I have just been reading Janie Franz's Edits with LY words which makes a lot of sense.