The article doesn't address Open Source. It states that the store will not be used for the distribution of beta versions of software. The tweet just says firefox betas won't be in the store.
Especially note: Free software is allowed.
There are classes of software that won't work in the store. The article mentions:
• Things that implement their own copy protection.
• Buggy software.
• Things that need root.
See the whole list: http://pastie.org/1236378 but you will go blind before you finish if your don't have readability plugin.
I would characterize the forbidden stuff as "obnoxious, useless, or malicious" applications. There is some collateral damage along the way, e.g. anything with a kernel extension, and there are some morality/legality driven restrictions e.g. encourge minors to consume alcohol or realistic human killing.
The most alarming exception to me, is that the application can only distribute updates through the store. I would hope Apple doesn't hold up new versions like they have with the iOS stores. As a developer I cringe when I read of customers living with month old defects because the update is stalled in queue.
Customers will have a reasonable belief that stuff from the store isn't tearing through their computer looking for email addresses or credit card numbers to sell.
But that's ok, they have other channels for distribution. It isn't a Apple Store or nothing proposition like the iOS devices.