Over at Dan's Data! Dan has written an article about what he's learned in the course of reviewing HSF's plus what makes a good one and titled it The CPU Cooler Snap Judgement Guide.

I've written this CPU cooler comparison. It is, roughly speaking, the size of a cow.
In the course of writing, and regularly updating, the comparison, I have learned some general things about CPU coolers. Since there are about a squillion coolers out there that I haven't even seen, it helps to have some rules of thumb to tell you whether they're likely to work well or not. You can't count on being able to find a decent review of every cooler on the market.
The pics and links in this review are to coolers that are in my comparison, but the following rules ought to apply to everything.