Easy, Kev. You've now completely hijacked this thread. (Let me join you.)
I was rather active in the user management discussions. In fact, I wrote two definitive blog posts on it. The second is almost certainly going to be the spec for when or if we decide to go forward.
So, a few issues. One, our feature roadmaps and our API roadmaps are two completely different things. Post formats, DFW, drag-drop uploads, and default themes have never affected our goals for development. Indeed, the primary people responsible for the backend code hardly touched any of those.
Building new APIs and handling complex schematic changes are grueling, and multiple core developers need to decide they have the stomach for it. That is why we haven't touched it yet. For as good as my spec may be, it is going to be an absolute nightmare to implement it (and certainly would have been even worse before PHP5). The backwards compatibility aspects are going to be one of the hardest challenges we have faced since 2.3. It's fine that it has sat there for a few years, untouched. One day, in the future, someone is going to dust off that blog post and do something awesome with it.
There is a huge difference between refining the admin bar — again, a feature — and trying to reverse 9 years worth of assumptions based on absolute URLs. It's a hell of a lot of code to rewrite while still keeping compatible. Also, Matt didn't blame that on b2. Two developers in charge of the backend codebase (me and Ryan) did.
If I turned around tomorrow and said I'm going to tackle some old, ridiculous "API" and make it better, and then succeed (the important part), guess what, It gets into core. But I need to have the stomach to start rewriting large swaths of the codebase with the hope I don't break anything else. (You may have already read this mailing list post from me, where I talk about the future architectural patterns of WordPress.)
And that brings me to the other point — in the nicest way possible, patches are welcome. I normally hate that phrase, because it's a total cop-out. But, countless aspects of awesomeness have made their way into the codebase because one person prioritized writing it, and core developers realized that person had something good.
My final point is, funny thing, you and I just spent a dozen or more paragraphs talking about user management, when in reality your comments + edit_posts bug has nothing — nothing — to do with any of that. It's a small, unrelated bug. We actually did mostly fix this in one of the releases you said we ignored the problem. In 3.1, we introduced an edit_comment meta cap, which gave a lot more power to plugins to make whatever modifications they needed to make to it fit their desired workflow.
And so all we actually have left is a simple bad cap check. It's one a plugin could work around, but here's the ticket for it.
I'm just saying, not everything is so complicated. :-)