I think those things fall under 'style' because while you can evaluate the mechanical aspects, and most readers will struggle more with incorrect writing and enjoy it less, the plain language -> lively use of metaphors can be invigorating or offputting (and well and badly done) - but some readers don't mind headhopping, sentences that go 'and then and then and then' - while others *do*.

So you can be objective/descriptive without making predictions about enjoyability or quality of the book.