Google uses an intricate system to score links up or down with regard to their power to influence its results. When blog commenting, you are looking for these qualities in a link-drop target:
1. Relevancy - Look for your keywords in the title, content and incoming anchors of the target site.
2. Page Rank of the target page (as opposed to the domain) - It isn't the metric it used to be, but it still shows node importance. Many people will try to tell you to disregard PR entirely but this is not wise. It's ancient but it's still the rusty old backbone on which much of Google's system is built. It's no good for telling how well a site will rank, but it is a good indicator of juicy targets from which you can sap link power.
3. The fewer outbound links there are on the target page, the more powerful your link will be.
4. The "cleaner" the page is, i.e. free from words like viagra, cialis, sohbet etc, the more likely it is that your link will get through Google's spam link evaluation filter.
5. Absence of the nofollow tag on your link - nofollow signals to Google that the link is user-generated content and cannot be vouched for. As a result, Google does not take the link into account in its ranking calculations and you've wasted your time.
There are a lot of other, smaller qualities you may look for but these are the main ones. They are desirable, but not essential. The more factors in the list above, the more powerful your link becomes.
Remember, Google is not usually in the habit of penalizing sites regarding where they get their links from. After all, who can control who links to them? The worst that is likely to happen is that your link will be disregarded.
