Mark Millar is not a bad writer. I think the things he chooses to write about are very well-written. He's good at the craft of writing. I don't think he's a good storyteller, though. Grant Morrison is a great storyteller. His sense of pacing and the interactions of his characters are both top-notch. Morrison can write some stuff that at first seems to have the consistency of a fever dream but he usually pulls an excellent and interesting story out of all of it. For example, Morrison's Zatanna mini-series really has me hooked. It's got a cool, well-told story and compelling characters such as the girl who is tagging along with the title character.
Millar seems to have an idea of where his story arcs are going but, to me, it looks like he gets lost along the way. He's got Points A and Z thought out but there's not much of a sense of the path between those points. Many things just seem to happen. He has dropped some hints about future events (such as the Vision android) but the introduction of said synthetic being just seemed incongruous.
In Ultimates 2 Millar is definitely going somewhere with Pym as well as with Captain America & The Wasp but I get no sense of an arc or of any compelling elements which tie any of the parts of this story together. I have no problem with a comic that tells a self-contained story in each issue but Ultimates 2 is obviously on an arc.
Another thing that good storytellers do well is suspense. Allan Heinberg is doling out the information about the main characters in Young Avengers with an eyedropper. And I love it. We are several issues into the series and we just found out something very important about Hulkling. Heinberg is keeping me interested and I actively look forward to each new issue. This is, no doubt, something Marvel would be thrilled to hear.
Millar, however, has not captivated me with the mystery of the Ultimate Traitor. Ultimates 2 #6 ended with another tease showing that Pym knows who the traitor is but we, the readers, don't yet. It also ended with a cliffhanger that in the hands of a better writer would have made me say, "Woah! What the hell? I can't wait to see what that's about." Instead, because the events of the cliffhanger have had no build-up, I don't care. That's something Marvel doesn't want it's readers to say.
The opposite of love is not hate; it is indifference.
I think the main reason that Millar's writing bounces off me is that he doesn't seem to care for the characters he's writing about. Allan Heinberg does. Joss Whedon does. Giffen and DeMatteis do. Let's take another writer who does.
Warren Ellis.
This allows me to make a direct comparison between the two writers because they both worked on The Authority. Ellis writes cynical, smart, bastards who have seen too much. Some say he writes those characters too often but that's fodder for another essay. Jenny Sparks and the rest of the Authority are willing to do extremely violent things to make the world a better place for the majority of its inhabitants. They are not amoral but they do not abide by any law that keeps them from doing their jobs. This is one of the core premises of The Authority.
As written by Ellis, The Authority were a bunch of bastards but they were Bastards for Justice. They were also sympathetic. There were several moments in the first twelve issues of that comic which moved me and one or two that nearly had me in tears. Millar never did that. When he took over The Authority became assholes. Just assholes. I no longer cared about these formerly fully-realized characters. And I don't think Millar did, either.
EDIT: The distinguished commentor makes a good point. I could tack on some specific examples of the differences between Millar's and Ellis's Authority work but it will take up quite a bit of space and this post is plenty long, already. So, STAY TUNED for part two wherein I go on at length about that comic book.
No one will be seated during the thrilling Hegelian Dialectic scene!