Atom's support for categories is unclear, of course, because that section of the specification has not been written yet. When Atom Protocol is complete, it will offer more, not less, functionality than the MetaWeblog API. Contrast that with the MetaWeblog API, where the spec is "complete" yet categories are still unclear.
Suffice to say that this does not fill me with confidence. Categories are not complicated. A post can have N of them (never mind that this server only supports one category per post :) ). A client has to allow some way to attach one or more categories. While I don't much care for the woefully underspecified MetaWebLog API, it at least takes a stab at it. They've been at Atom with the hammer and tongs for a few years now - if something as basic as categories causes confusion, then I have rather large doubts.
I expressed concern over the base64 inline attachments that Atom allows - to which the answer comes back:
No, that is not the Atom Protocol solution for Podcasting. To reference a Podcast from an Atom entry, you'd use the Atom Link element.
Seems to me that by having both, some people will do one, others will do the other. The inlining is a disaster waiting to happen. I can see it now - I'm traveling, and I've found a hotel that only has dialup. I start my aggregator off getting updates, only to find out that some puzzlewit has inlined a 10 mb file. Oh joy - I get to enjoy downloading that until it clears his feed, because - even with a smart HTTP library, the entire thing will be "new" each time a new post gets added.
Having said that, the Atom posting API looks mostly rational. The trouble there isn't with the spec, but with Blogger (Google) having rushed out with support for the 0.3 feed spec - and now, apparently, support for gosh knows what intermediate version of the posting API. This doesn't simplify my life :)