I always tell people my skill level first. Someone got upset at me cause I was giving people advice without telling them my level of ability, so now I put that in just in case.
I usually just put down what I like, and if something is bothering me, what I don't like. I try not to get into specifics. A person of my skill level really has no base telling people what's specifically wrong with their photos, unless its obvious (like cut off limbs, rule of thirds, etc.). But I do feel confident enough to say that I like certain aspects of it. And hopefully my insight into why I think the photo works, will help them understand what aspects of the shot make it look good.
The best advice I get is when people tell me how to achieve a desired look. For example, I just had a thread where I over sharpened a photo. I learned how to tell if I had over sharpened it, and also what steps to take in order to insure it doesn't happen again. It was some AWESOME feedback.