This post is aimed especially at noobs who's first post on DP is asking for a web site review, but it applies to everyone. If someone takes the time to leave feedback about your site that is truly useful, the least you can do is reward them with a positive rep. You don't need to rep someone if they leave a post that says "Looks good," but if they give you a useful suggestion (especially one that you eventually implement), just let them know that you appreciate their time and effort!
What I'm afraid of is people who decide that my reviews aren't useful because I don't really tell them what is good.
As long as you are constructive in your criticism, I think the majority of people will not take offense. When you ask for a review of your web site, it is important to accept the good along with the bad.