Libertine Seguros said:
That's terrible. Must be pretty traumatizing for the competitors on track with her as well. While as ToreBear notes, this kind of thing does happen occasionally, and as was discussed on the forum back when Fabrice Muamba had his on-pitch collapse, athletes are at higher risk of sudden cardiac problems than the general population, it doesn't make it any less shocking or surprising when it does.
Heidi's still never won a World Cup race. When you look at some of the people who have, that's pretty crazy. That's part of the price you pay for being good at distance these days though; the field is not deep enough to allow for proper distance specialists, and those races don't allow for the factor of luck to the same extent as the sprints. It feels like for most of the season, the mass start distance races have basically been Marit leading a chasing duo of Therese and Heidi, then everybody else way back. Obviously the Worlds are a different matter with different people peaking, and Heidi is still plenty young enough to have future opportunities, maybe she just messed up her timing of when she peaks.
Heidi still has yet to figure out the peaking thing. Usually she just gets better as the season progresses and is at her best in March.

It might be she and the coaches have figured this out this year, or she will just keep improving through the season. I think it's part of development of learning how your body reacts to training and racing.
As for yesterdays performance, to me it looked like she had terrible glide on the classic part. One of the things that make Marit so great is that she has an enormous amount of skis for different conditions. Heidi is still too young to have amassed such a good inventory as well as the experience in racing in various conditions. So with the selection of skis not fitting the conditions it's not hard to imagine her struggling in yesterdays conditions.
It could of course be she has suddenly dropped out of form, but in my mind, skis/condition/technique were her issues yesterday.
As for Marit, it just seemed like she had a really bad day. She has elected to not start in the sprint relay to get some rest.
As for Heidi not being on the top of the podium, I think she is close, but Marit being the best female skier of all time, I think second to her is a victory in it self. Weng is the only one I currently see being able to beat Marit in a sprint after a long race.
In a purer sprint, Stina Nillson is the one with the best chance to beat Marit.