Title
The most important thing you can do to your ranking is to include the keyword in the title you submit to Yahoo. If your title does not contain any keywords, you'll never rise high enough to collect a large amount of clicks. Is including the keyword once in your title enough, or should you risk it and try to repeat the word? I didn't see a single site that had two or more instances of the same keyword in its title. That doesn't mean there aren't any, but I don't believe that having two keywords will give you a big advantage over having one. Also, trying to repeat the keyword might get your title edited or your submission rejected, so I wouldn't try it.

Description
This really surprised me. I started the experiment with the belief that having the keyword in the description is absolutely vital in order to gain a high ranking. However, the data seemed to prove otherwise. The top ten sites actually had less keywords in their descriptions than those listed after them. Still, I don't believe that including keywords in your description will actually hurt you. It is likely to boost you upwards a bit, but I think the boost will be significantly smaller than what it has generally been claimed to be.

URL
No clear data here, so I won't make too strong conclusions either. Some words are in order, though. My current position is that having the keyword in your URL is worth something, but less than having the keyword in any of the other places (description, title, category). Is it necessary to get a keyword rich domain name if you're desperate to reach the top ten? Not in my opinion, unless it will give you a valid reason to include your keyword in your title.

Category
Being listed in a category that has a keyword in its name definitely increases your ranking for that keyword. More than having the keyword in the URL and more than having the keyword in your description, but less than having the keyword in your title.