When you are choosing a domain name, how specific do you make it? Does it, without fail, describe what the site is about?

Do you find that making it so descriptive reduces the ability to later broaden out with your product?

If you have a good business plan your domain name will reflect that.
Short answer is that the more descriptive your domain name is regarding your product the better return on your dollar.

Sure random domain names can work out well, ie : Yahoo/Google but most people don't have millions of dollars to brand those names.

On of the most important things is for the name to be reasonably memorable and pronounceable. I've learned that thing from my own mistakes. With the requirement to keep domain names short, you can't go much into the description part anyway. Product names can be more elaborate though.

Theres no set rule that domain names be short but it is recomended to have it be less then 5 words and even that is a lot.

If you tend to make your domain name more then two words long make sure that it is at least memorable

If you are choosing a domain name based on your site, consider a domain name with keyword.

Including a keyword's fine, if it makes long term sense, but using a hyphen should avoided.

Yes, I can see using keywords would make a lot more sense than other things, and in fact that could circumvent the problem of using either a product name or general name-because if your products are all related then a couple of keywords in a phrase could have you covered.