Here is a very interesting article (Androinica) about usage of Android Market comments.
The summary :
1. Rate only the app, not the subject of the app.
2. Stop hijacking stars for non available features with comments like ‘4 stars until you add xxxx’
3. Don’t seek tech support: Devs can’t solve a problems with a comment of 1 line. We need to exchange ! so, email the dev.
4. Don’t download just to trash talk.
5. Don’t be a jerk