There's a difference between ranting and a thoughtful critique. I'm sure you didn't mean to sound quite so rude, but that is how it appears. I saw this happen quite a lot in the university freshmen level art critiques, but they tended to disappear by the sophomore and senior years. So perhaps it's just something that comes with age and experience. Here's a list to help you in the meantime.
1. You do not have to be a talented artist to critique someone's work, but please know what you're talking about. An uneducated critique will sound ignorant and childish. I think you did well in this area by giving examples as to how to correct certain areas. It is the next two parts that you seem to have trouble with.
2. You do not have to like the artwork you're critiquing, but there is no excuse for rudeness and bad manners. You SHOULD NOT make a simple list of everything you think they are doing wrong, and then post it on their picture. This is bad etiquette. Critiques should contain a balance of things you believe they are doing successfully, along with some things you think can be improves upon. The balance is very important. So, by this definition, you did not provide a critique. You provided a rant.
3. There is no need for a thorough critique if the artist does not ask for one. This should be more common sense than anything. Maybe the artist knows what needs work but simply does not have the time to work on it anymore. Pointing out what they already know will only dishearten them in future work, aka: it's not helpful at all. By posting your rant (which was uncalled for) you did only harm to the artist, and to your reputation.
I hope you take some of this list to heart, and it will help you in your future attempts of being a respected colleague in the art world.