Immortal Iron Fist may have been one of the first books I bought primarily because of the creative team, rather than the characters. Granted, when I first heard about it, I somehow confused Ed Brubaker with Brian K. Vaughn, who was writing Dr. Strange: The Oath, which I was very much enjoying. But the point remains, I don't recall having any strong feelings one way or the other about Iron Fist prior to this title, yet, here we are.
One of the major thing Brubaker and Matt Fraction did was posit or explore the idea Danny Rand was only the latest in a long of Iron Fists. Even if DC went away from the concept in the immediate aftermath of Crisis on the Infinite Earths, the notion of characters being part of a legacy or tradition made a slow but steady comeback in the late-90s, spearheaded by Starman and JSA.
Marvel never seemed to have a lot of time for that, outside the occasional nod like Rick Jones trying to take the codename Bucky to be Captain America's sidekick. Marvel seemed to prefer the "replacement" approach. Jim Rhodes taking over for Stark as Iron Man when Tony was in his cups, or Scott Lang becoming Ant-Man after Pym had shifted to Yellowjacket. When they did explore it, it was usually relegated to side universes like Spider-Girl's MC2 Universe.
After Immortal Iron Fist's success, suddenly legacies started popping up for all sorts of second-tier characters. Jason Aaron used it on Ghost Rider, and Moon Knight is nodding to the idea right now. Immortal Iron Fist also expanded the mythos what I would call laterally, by adding another six "Heavenly Cities", each with their own champion, with Fat Cobra seeming to be the fan favorite of the bunch.
(Aaron's Ghost Rider did this too, with Spirits of Vengeance across the world, and MacKay introduced the notion Khonshu has two "fists" at the same time.)
Amid the larger story of Danny learning he's really just the most recent (and least well-prepared) Iron Fist at the exact moment he's besieged from all sides by both HYDRA and his old enemy the Steel Serpent, the book would take time out to devote one-shot issues to some of the previous Iron Fists. Issue 7's spotlight on Wu Aoi-Shi, the Pirate Queen of Pinghai Bay (drawn by Travel Foreman, who also drew flashbacks at the start of most of the issues, as well as Leandro Fernandez and Khari Evans), seemed to be the fan favorite.
David Aja was the primary artist, and he brought a different sensibility to the action sequences. Where other artists would combine multiple actions into a single panel (via the after-image approach), Aja would sometimes draw out the sequence of Danny jump kicking a single HYDRA agent through a subway window across three panels. Really stretch out the moment. A lot of alternating between larger panels to show the scope of the challenge and lots of small square panels to cut between important points within a scene rapid-fire.
He also ditched the '70s high collar with plunging neckline costume early on in favor of the number above. It's more streamlined and practical and I don't have any complaints, but I don't know how more committed Iron Fist fans felt about the change.
Unfortunately, having David Aja as your regular artist means that if you're going to keep a monthly schedule, you're going to have fill-in artists. By the latter stages of the "Capital Cities of Heaven" arc the covered the second half of their run, Aja was sometimes drawing only a handful of pages, or none at all, with Kano or Tonci Zonjic taking over the lion's share of the work. They were close enough in style that the character's maintained a similar look, but they didn't seem to have Aja's sense of style when it came to page layouts.
Also, the arc suffers from either not enough time, or too much build-up. We're told what's at stake is how often the Heavenly Cities may intersect with Earth. The loser, rather than being limited to once a decade, will be limited to once per 50 years, for example. It's unclear how you would determine a loser since there was apparently to be a battle royale between the fighters who lost earlier in the tournament. For that matter, if Orson Randall fucked the whole thing up last time by first refusing to fight, then killing another of the champions when he was to be punished and fleeing, how did that not result in K'un-Lun getting barred for fifty years?
It's a moot point, since the whole notion is dropped entirely to focus on the HYDRA subplot, which is itself really a revenge plot by a new character, descended from another new character we meet in flashback. Overall, I'm not sure the Brubaker/Fraction/Aja run doesn't end up being less than the sum of its parts, but the book didn't end when they departed.