Writing precisely and keeping it laconic is a daunting task. The best way to track the length and the content of your writing is to plan your thoughts in advance and only then let your fingers type it.
Know your Audience
A short shrewd feedback is very powerful. Still being short it can give a lot of insights. A reader reacts immediately to such things. Noone is interested in reading the same thing. An editor must be independent in their thoughts.
Content is A King
Be your own voice.
Adjectives: Extreme adjectives. Adverbs. Simple but good grammar. It is enough to make your feedback tasty and appealing to an eye.
Logic is Power
Keep close to a text or picture you are analyzing or describing. Don't twist the meaning. But it is good to exaggerate it a bit. Exaggeration adds up some mood. In a moderate manner.
First and Last
As a matter of fact, the last sentence of a feedback makes this positive or negative impression. The first sentence sounds like an intro. Don't confuse it with an essay.
Get your Credits
Garnish a feedback with some explosive turns.
This trait of a character may not be your strong side but Evernote, where you mostly leave your comments, makes it possible.