A common complaint I hear about feedback on someone's writing is that it focuses too much on stylistic changes (grammar, word choice, word order) and lacks good suggestions about improving the content or the message of the work. This is likely because it's much easier to comment on style than on content.
To prevent myself from doing this while giving feedback, I have begun following this general rule: if I can't provide a good, clear reason for why I am suggesting a change, then the change is unnecessary.
The only problem with this rule is that sometimes a paragraph or sentence really can be too difficult for a reader to understand because of its vocabulary. In this case, a stylistic change that simplifies the message is warranted.