The way I see it, there are four categories of comments. I've gotten all four, sometimes on the same rotation, showing how random they are.
1. The honestly really impressive great comments. These include: I hope (s)he chooses to go into this specialty. Had the most impressive fund of knowledge for his/her level I have seen in years. Was a real advocate for his/her patients. These are rare, and if you get these, you should feel lucky.
2. The good comments. These are comments that, if speaking to a lay person, would sound great, but really just meant you did okay. Most comments are in this category. Well-prepared. Excellent fund of knowledge (this one's on the high side). Good student (on the low side).
3. The bad comments. These are the comments that sound good, but because they are so blase, are really slightly negative. Adequate fund of knowledge. Improved over course or rotation. Quiet.
4. The really bad comments. No explanation needed. Appeared to struggle on ___ clerkship. Below average fund of knowledge.
It's kind of like grade inflation, except more random. Just my opinion, of course.