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Going along with what DG suggested, it's word order and proximity also. How many times the words are used together in a phrase, what order they're used in, how close to each other, and how they're distributed throughout the page.
I've seen one person's page get slammed down 40 places in the SERPs using excessive repetition - density 30%+ - and yet seen other pages do fine with that much.
Generally when someone finds what they think is the sweet spot for density they're not gonna tell *anyone*, and it might not even apply to other sites. I've got what seems to be working on several of mine - yet it might not work for someone else because everyone puts pages together differently.
Also, there are other factors that come into play - inbound and outbound link text, and contextual relevancy for the linked-from an linked-to pages.
It takes a little while to work with a given site or page for density, and checking the ratio of density between phrases on the same page. If something needs tweaking up or down, it's a gradual process, making the adjustment and seeing the effect.
I just watched one page drop down 3 spots, directly attributable to the loss of only one occurrence of inbound link text - and changes to the page's density downward made without telling. It's a matter of keeping track, and with the *Fresh* spidering, seeing what effect another density change will make without any difference in links or PR being factored in.