Forbes, Financial Times, New York Times
Edit: I feel like I should explicitly state a point here. Between the live front page and the one you linked to the live version has more mainstream media. But it always seems like it's on the decline, whether it is or isn't.
LinkedIn, for example, routinely offers business articles from the same sources that have been appearing on HN more frequently. In fact, it's not uncommon to see an article shared by LinkedIn make its way to HN shortly thereafter. (The correlation/causation dynamics are unclear, however, so we should keep that in mind).
extremetech is very annoying to me at the moment. Some of the articles are vaguely on topic for HN, but the coverage is usually well below what could be appearing on HN. I would much rather people link to the original source than to places like ET or The Register or the aggregators.
EDIT: Has anyone created an HN stats software? It might be interesting to see graphs of sources; votes; comments; etc.
But I'm sure as "wheels" pointed out on the mainstream perception, the ratio was the same when I've signed up years ago.
I post rarely, sometimes from NYT, and try to contribute articles that are germane, inspiring - not retellings or layman's gee-whiz reporting.
Overall, I don't think this is an important problem for HN - if a problem at all. And of course, we can change it ourselves at any time.
I'll let other users here check my user profile, including its extensive list of submissions, and then let me know how I fit into the community here by how I submit articles. On my part, I agree that HN has long been a place where one can find stories that aren't reported elsewhere, and that can be both a feature and a bug. HN is an especially good place to find breaking news on its core topics (?) of start-up business and new applications of software technology. It is also a good place to find thoughtful discussion on stories from a variety of sources about the impact of technology on society, education policy in light of technological changes (what drew me here), and the personal development of hackers (what keeps me here, as the parent of four children who may all become hackers). I love the thoughtful comments here, even on metadiscussion topics like the issue of "What's on topic on HN?"
That said, I don't find it bad to find story sources here that are "mainstream," or sources one could find elsewhere. We still won't see the same kinds of comments on those stories elsewhere that we can find here on HN. Indeed, compared to many of the blogs that are submitted as sources on HN, I would MUCH rather see more stories from the Wall Street Journal, the New York Times, National Public Radio, Time magazine, and The Atlantic, simply because those are all professionally edited sources that often (not always, but often) do a good job of checking facts and establishing context before publishing a story. I especially don't like submissions of stories here from linkspam blogs that quote a paragraph or two of a professionally reported and edited story, add a more eye-catching headline, and do nothing to add value to the story. If there is some breaking news that is first reported by the mainstream media and on-topic for HN, link to the mainstream source. Don't reward lazy bloggers with eyeballs.
As examples of sources that are not mainstream, so far not frequently posted to HN by anyone other than me (usually), and well worth reading when they have articles on-topic for HN, I suggest
for breaking news in biology,
also for breaking news in biology, with especially good reviews of gee-whiz stories that make it onto HN from worse sources,
for general overviews of scientific reasoning,
for reviews of new developments in medicine and biomedical research,
for reporting on education policy, especially science and mathematics education,
for interesting news about mathematical research.