Any time I re-read David Copperfield, I'm reminded of something that is so widely accepted that it's practically crossed over from opinion into semi-fact (or "fact-esque" as Stephen Colbert would say): this novel has one of the worst-ever cases of second-act troubles. That is to say, at roughly the halfway point or perhaps even a little earlier, the novel runs out of steam, loses momentum, and crumbles into a series of low-energy vignettes. Long novels usually get weaker toward the end, and especially the very end, where lots of plot threads have to be wrapped up, and the process can feel mechanical. David Copperfield extends that mechanical process over hundreds of anti-climactic pages, wrapping up one plot thread after another: will Uriah Heep get his, what's going on with saintly old Dr. Strong and his uncomfortably-young wife, where's Little Emily (who cares?), and so on. David becomes more or less a supporting character in his own story, standing by and observing the resolution of all these storylines; he hardly takes any positive action at all in the last half of the book. You could argue that that fits David's change from being a put-upon child to being a journalist and author -- he's more of an observer now -- but it seems more likely that it's Dickens losing interest in his own alter ego once he becomes an adult.
There are some potentially interesting story threads involving the grown-up David, but Dickens drops them very quickly. There's a chapter showing how the worthless, selfish Steerforth, whom David idolizes, gets David drunk and disorderly and hanging out with bad companions -- but this is the last inkling we get of the possibility that David might be led astray by his relationship with Steerforth. He doesn't even make a real, conscious decision to avoid Steerforth's influence; he gets a warning from Agnes, the most insipid heroine in all of fiction (or as Orwell called her, "That real legless angel of Victorian romance"), to stay out of trouble, and he does, but he doesn't lose his illusions about Steerforth until later. Instead of letting Steerforth come anywhere near close to ruin David, or forcing David to reject Steerforth, Dickens has Steerforth ruin the lives of David's friends and go out of David's life forever -- he doesn't appear in the novel again until his oh-so-convenient death. A good hero has to make moral choices; Dickens seems afraid of letting David make any such thing, or actually letting him confront the fact that he's chosen the wrong friend.
And the same goes for David's predicament of having chosen the wrong wife, the "child-wife" Dora. The dilemma of what the hero does when he realizes he's married a woman who, while sweet, is totally wrong for him -- that's a very interesting dilemma. The fact that everybody, including Dora, understands that David is better suited to Agnes (though Agnes would undoubtedly only be fit to marry a plaster statue) makes it still more interesting. But no sooner does David realize that he and Dora are ill-matched, then Dickens pulls the death card and has her fall ill and die for no particular reason. Apart from the general creepiness of killing a character off just so your hero -- who happens to be based on you -- can marry the perfect woman, Dickens is once again setting up a real problem for his hero and then immediately allowing the hero to avoid facing up to the problem. So the second act of "Copperfield" is weak, but it didn't have to be; it's Dickens' apparent compulsion to keep David's life problem-free that makes the novel go downhill. It's not a coincidence that the best parts of the novel are the ones where David is the most miserable.
I suspect Dickens learned from the less-interesting second part of "David Copperfield," in that all his subsequent novels have certain tricks to keep the reader occupied in the second half. "Bleak House" has that whole murder mystery to keep the second half interesting (the first half revolves around the mystery of Esther Summerson's parentage; once that's solved, Dickens introduces another mystery); "Little Dorrit" pays off certain mystery elements from the first half, and plays up the Mr. Merdle story more; "Great Expectations" keeps the hero front and centre throughout instead of letting him fall away. All Dickens' novels have second-act problems to some degree; it's a natural consequence of the way he wrote and the falling-off of inspiration that occurs when he has to start wrapping things up. But he did get better at second acts, which is part of the reason why his work of the 1850s and '60s probably stands as his best work.