Ender's Game has spawned a number of sequels since Card wrote the story back in the early 1980s. The sequels fall into two groups: one set follows Ender's life, and the other follows a number of the characters who surrounded him in the original novel.
I've read or am reading all the sequels. None of them has been as compelling as the original. It took me a while to realize that, and even longer to understand why. It was only an hour ago, as I was reading Ender in Exile, the most recently published sequel, that I put my finger on the difference.
In Ender's Game, Ender's own thoughts form much of the narrative. I love the claustrophobic atmosphere this creates. It puts the reader in Ender's shoes much of the time. The story focuses on what is important to him and lets the reader imagine many of the details that must have formed the totality of his perceptions. Less obviously, there is a straightforwardness that keeps the narrative moving and a nervous tension in Ender's point of view that keeps it gripping. The reader comes to identify with Ender and to care about him.
The sequels lack the intensity and focus of Ender's Game. While the stories they tell are clever and there's a lot more plot, the point of view is more distant. Some of the difference between the first book and its sequels is unavoidable: Ender's Game is Ender's story, while the sequels, even the ones that include him, tell stories that are larger than him. However, the sequels also suffer from a change in Card's writing style.
Dialogue is limited in Ender's Game. Much of it is utilitarian and uninflected: Card doesn't often tell you how to interpret it with adverbs ("he said icily", for example). Also, much of it takes place in passages that open, but are separate from, each chapter. The passages are conversations that put the chapter's events in a larger context for the reader and convey information Ender does not have. In contrast to the conversations between Ender and other characters, these isolated dialogues are often lengthy, and the speakers are sardonic, sarcastic, and otherwise clever. Everyone in the Ender novels is smart, but in Ender's Game only the participants in the chapter opening dialogues are allowed to banter at length. In the sequels, Card abandons the terseness of the bulk of Ender's Game and lets his characters talk. And talk. And talk. And talk. And talk.
I like clever repartée: you can't be a fan of P. G. Wodehouse and not like clever repartée. But Card isn't Wodehouse, and Card's characters aren't caricatures. At least, they're not supposed to be.
What they wind up being is mouthpieces through which Card can be witty. Initially the banter is fun, but over time the sameness of the rhythm is wearying. Worse, the characters never become fully fleshed out. They drive the plot, but they don't come to life: it's impossible to care about them. I kept reading just to see how the plot worked itself out. That's not necessarily a bad reason to read something: The Hunt for Red October is a pure pleasure in that respect, for instance. The trouble is that Card set up certain expectations with Ender's Game, and I was disappointed to find him working different territory, stylistically speaking, in the sequels. Plus, The Hunt for Red October has a much more compelling plot than any of the Ender's Game sequels. Finally, Card wasn't aiming to write tech-driven thrillers: the sequels require that you be emotionally invested in the characters (see, for instance, Miro). Unfortunately, they're too busy talking cleverly to be real.
I haven't read any of Card's works not set in the Enderverse, so I can't say whether the Ender sequels are characteristic of his style. If they are, we have to regard Ender's Game as the luckiest of accidents. In that singular work, Card fused a compelling story and an even more compelling storytelling style to produce a terrific book. Nothing (much) wrong with the sequels ... but nothing so ineffably right, either.