I'll try to do a more thorough review when I read it more thoroughly but I have a couple things I can relate after a single read-through.
I love, really *love*, the mid-sized story format. IMO, full book adventures often get more detailed than they need to be because of inevitable GM customization, and thus material gets unused. On the other end of the spectrum, I've seen SR books with very short story seeds that are great if you have time to develop them but really don't provide enough material. The format in AU is just right. The format gives me, as a GM, just about the right amount of material to let me fill in the gaps and customize it to my PCs but without making me do all the heavy lifting.
As dramatic and potentially metaplot-altering as the Praxis section is, it is extremely unsatisfactory to me compared with what I know about the original direction this story was heading in the unfinished Harlequin's Gambit adventure.
The idea of the PCs running around using the artifacts to unlock caches of knowledge and goodies from the 4th Age in a big game of chal'han between Jane Foster and Sheila what's-her-name from the Atlantean Foundation is, frankly, awesome. Also, unlike Praxis, the PCs actually get to participate in the big events. This is not to say that the mid-size adventures in AU aren't interesting and fun; many of them are. It's just that they seem like sideshows to the main event and the main event has no room for PCs to be involved. There is no satisfactory culmination for the players who have played through the entire DotA series, no sense of climax after the long build-up of gathering all the artifacts. They just separate and the PCs are left chasing some second-order unintended consequences of the artifact chase.
The other problem (for my definition of problem) with the AU adventures is that most of them proceed from the premise of the artifacts "not wanting to be together," which I think is a pretty much crap idea. Sadly, my group won't be running through much, if any, of AU because I'm definitely going to use the HG draft as a basis for a capstone adventure for my campaign.
I know this post won't be terribly helpful to the individual authors of the mini-adventures, as it addresses big picture issues none of the authors likely had much say over. Still, I think it needed to be said and I'll read the adventures more carefully in the near future to give better feedback on those.