I don't want us to get waylaid by disputes about novels not under discussion, but since you invoke The Child in Time—and cite it as a model of what a good Ian McEwan novel can be—let me explain briefly why I didn't much like it. This might shed some indirect light on our present disagreement.
The Child in Time takes as its premise perhaps the most horrific event a parent can imagine, the abduction of a small child. An idea like that is a guarantor of emotional power; no special skill is required to make the situation resonate for the reader. I don't mean to say it's necessarily a cheap idea—McEwan is never cheap—but still, it does at the very least buy its dramatic potential at an awfully good price. What follows, though, struck me as more the working-out of a literary conceit than an honest exploration of emotional consequences. Now, I read the book 15 years ago, so my memory may not be completely reliable, but as I recall, the novel's turning point consists of the extraction of a lorry driver from his crushed vehicle after a motorway accident. The description of this process, jaws of life and all, is executed in such a way as to resemble the forceps delivery of a baby. You know, a symbol of a new life starting and so on. To be honest, despite its cleverness (or rather, because of its cleverness), I was offended by the scene, and by its place in the author's dramatic schema. It seemed unworthy of the novel's potential; it seemed to bespeak the wrong sort of ambition. A little less art—or at least less self-conscious, less arty art—might have better served McEwan's artistic aims.
Whereas Atonement, for all its … you call it "showing off," and I won't cavil at that. But for all its virtuosity, and its manifest reveling in its own virtuosity, Atonement strikes me as a more emotionally honest book, truer to its premise, less wedded to literary effect.
I'm surprised you found the opening section dull. The pace is unquestionably slow, but then, the same may be said of Mahler's great adagio movements. The pace must be slow precisely because so much is happening, a great deal of it below the surface, and because every scene is informed by a multiplicity of tensions maintained in perfect equipoise. The magic of that 37-page interruption you scorn is that it isn't simply an interruption; it brims over with its own vivid dramas, with the poignancy (and understandable horribleness) of the Quincey children, with the lies at the heart of the Tallis marriage, with Cecilia's uncomprehending erotic anxiety, with the arrival of her beloved bland older brother and his bland but somehow still creepy chum, with Briony's narcissism and confusion and pre-adolescent hysteria. And, of course, with our knowledge that Robbie's obscene love letter is going to be delivered and read at any moment, a novelistic analogue to Chekhov's loaded gun, waiting to be fired. All of it held in exquisite, elegant balance.
I'm also surprised that you don't even mention the novel's second part, taking place in 1940, in which the wounded Robbie and two fellow tommies exhaustedly trek through the French countryside toward what seems like the desperate fantasy of evacuation at Dunkirk. If death, as you say, is McEwan's power alley, then it's hard to believe this densely rendered slog against death somehow failed to grip you. The surface is rich with incident and vivifying detail, and the subtext, with its class tensions and the ever-present (although superficially absent) weight of the main plot as it presses down on the beleaguered Robbie, is full of the subtle contrapuntal drama of which McEwan now seems a complete master.
But let me end this posting with a leap ahead to a section of the book about which you and I may actually agree. For all my enthusiasm, I do think the epilogue is a bit of a miscalculation, a piece of cleverness that's too clever by half, reminiscent of precisely those qualities I've resisted in earlier McEwan novels. Again, I don't want to give too much away, so I'll restrict myself to saying this: The epilogue implicitly suggests that everything preceding it is itself an element in the story it purports to tell, as well as a grand gesture providing the book its title. I found this sort of circular structure distancing, alienating, a device calling far too much attention to itself. It breaches the fourth wall without adequate justification and without sufficient payoff.
The damage done is relatively slight, though. Even if it keeps Atonement from absolute perfection, the novel still represents, to me, Ian McEwan's arrival as one of the major writers of our time. I'll happily settle for that.