Some key words will appear naturally over and over in web pages. For example, on a poker website, the word poker will appear dozens of times on a page... it's nearly impossible for it NOT to happen.
I think Google and the other SE's take that into account.
Hidden text is a whole different animal - definitely blackhat
seo.
Meta tag stuffing - I think we all do it to some extent - that's what they're there for! But - only with relevant keywords!! I know when I see a site that has weird description text in Google, I pass it and visit a site with a description (meta tag) that makes sense.
Here is the keyword count for "poker" on one of my sites:
Title: 1: 7: 14.3%
Body: 97: 1285: 7.5%
Links: 63: 309: 20.4%
Bold: 30: 226: 13.3%
Meta description: 3: 33: 9.1%