Critique happens. This is the first thing everyone who posts a story needs to be aware of. Not everyone is going to like what you write. They could show it through a lower rating, or even an unexplained and undeserved hater-rating. The most useful way to deal with offering a critique is a comment as people have been saying.
If you get a tip or a critique on your writing, whether the commenter flagged it as such or not, do not panic. None of us likes negative feedback. We would all like to think that everyone thinks we are brilliant (even when we ourselves are not so sure). Sadly, it is not the case, nor should it be.
In posting a critique, the person has proven that they a) probably read your story, b) actually thought about it, c) cared enough to say something. There are going to be times when someone says something thoughtless. Feel free to ignore those particular critiques. Luckily, Protagonize is an amazing community and there are tons of helpful people here who want to help you improve your writing.
Here are my steps for dealing with a critique (I use this for feedback on my academic writing too):
1. Read it. This seems obvious, but read it carefully. Do not skim it because you may misinterpret what is being said.
2. Read it again, just to be sure you understand what was said. If at this point you find yourself nodding your head, skip ahead to #4.
3. If you found yourself feeling a bit hurt or confused by the critique, go do something else. Read a story, write, clean. Come back later when you have had some time (24 hrs is really safe, but that's often more than required). Then read it again.
4. Once you understand what was said, reread your story if necessary, especially if it is a specific point the commenter made (X line is a problem, verb tense, abuse of adverbs, etc). Try to be objective about whether the commenter is right.
Most of all, it is up to you whether you use the critique or not. Critiques are a chance to get someone else's point of view on your writing. They may not be right, you may not agree. Writing is for you. Then again, writing is also for your reader. This is a chance to find out how your reader reacts to your writing. Every critique is a chance to improve. Improving our writing is part of the deal. The best authors admit that they are always getting better. I look at it this way: I will be a finished product when I am dead. There are always things to improve, little or big, but it is often hard to see them ourselves. Critiques show us what did not make it clearly from our head to the page or screen.
So if you get a critique or a tip, take it as a compliment. The person thinks enough of you to want to help. They could have just said nothing.