Keyword Density Parameters?

On page factors in Google.

[Reno]

I run various pages through online keyword density analyzers every now & then, and have noticed that different tools will give slightly different results, depending on what the user sets.

Does anyone know what Google considers important in "keyword density". For example, <h1> tags may count, but meta = keywords may not; or italics may count but comments may not, etc.

My second question is if the meta keywords tag is now considered to be pretty much useless, is it also fair to say that meta = keywords will not work *against* you? So even if a person stuffed it over & over with the same keyword, from Google's perspective it is not even there? (so there is no penalty).

Thanks for any feedback...