> Badly formed XML? Check. There might be badly formed JSON, but I tend to think it'll be a lot less likely.
The problem with XML is mostly that it is a very complex format so the bugs are more probable and there are more pitfalls.
> Need to continually poll servers for updates? Miss. Without additions to enable pubsub, or dynamic queries ...
They actually did add tags to enable WebSub (previously called pubsub) so there goes that. For the other concerns, I think it is not the formats job to care for partial or incomplete data. Nothing prevents you to have a dynamic link with a "updatesSince" on your webpage and serve all of the articles that were added or updated after that. Nowhere, the format specifies the limit on number of items. It also incorporates paging out of the box so you could bubble up any old articles.
> Complete understanding of modern content types besides blog posts? Miss.
The point of this is for the open web, by definition nobody can anticipate all formats. Rather than fill the spec with tweets, facebook and other types, they have opted for the least common denominator and added a specific way to add extensions. This makes way more sense.
* An understanding that separate but equal isn't equal. Miss.
Nothing actually prevents you to leave the content fields blank and rely on the reader to pull the format. But for this kind of usage there are other methods. Personally I prefer content delivered in the RSS precisely to avoid to have to deal with customization of content formatting. JSON feed HAS a way to specify formatting though, it's called HTML tags. No need to reinvent the wheel here.
There might be mis-use and abuse, but if you want to avoid that you can always push markdown into the "text" representation.
