Keyword Stuffing... when does it become a sin?

When does keyword stuffing become a sin? We know that a lot of websites are doing good by stuffing a few keywords here and there, but some others are doing very bad and penalized big time by Search Engines.

Is it OK, acceptable or NOT OK to stuff your main keywords in:

1) Keyword in images ALT tags
2) Keyword in anchor link TITLE attributes
3) Every single paragraph of your texts. (Making the density over 40%)

 

I would say that stuffing is always a sin, whereas appropriately placing is not one.

 

Keyword stuffing is always a sin when your just inserting lot of keywords that give less value to your content.