For example, a link to Salon or slashdot doesn't count, but a link to a specific article does. The newest items should be at the top of the page. Old items should be rolled off to keep the blog page small. It should contain some commentary indicating what the link is and preferably also what the blog author thinks makes it worth looking at. Blogs are neither commercial products nor do-gooding nonprofits.
Blogs can verge on being commercial products, or the reverse.
I suppose that's like asking when does a journal become a fanzine become a magazine. Probably when enough people look at it. Blogs certainly can be slashdotted, and they get looked at a lot more when that happens. A big difference between a blog and a regular periodical, whether clipping service, journal, or magazine, is that blogs don't have to be published regularly. Many of them are mostly daily, but if the author is away from the net, hasn't found anything interesting, or just doesn't feel like it, there's no obligation to post anything on any given day.