Some of my writing friends asked me about what I look for when I read slush for Baen Books; especially, what makes a book a good potential fit for the Baen line.
Before I answer, let me point out that if a manuscript is at all close to being publishable, I pass it to the publisher, Toni Weisskopf, and let her make the call. She instructed me early on that she only expected me to send her the top 1% of submissions, but I've actually sent her more than that. Some I've only marginally recommended, if I thought they were good books but might not be quite right for Baen.
That being said, I don't know if this characterizes any kind of Baen "profile," but I look for:
1. Stories that are adventurous and fun at their core. Think "golden age of SF."
2. Stories that are exciting, if not actually action-packed. (Lots of people believe a Baen book must have a battle, brawl, barfight, gunfight, knifefight, or fistfight on every other page. That's not entirely true, but it's not entirely false either. Baen fans appreciate action; and what's more, they know well what makes for realistic action and are ruthless about inconsistencies.)
3. Stories that make sense -- e.g., with science, economics, etc., that ring true -- and are internally consistent. (This requirement is quite clear in the Baen guidelines.)
4. Stories in which characters' actions and the consequences of those actions make sense and seem plausible.
5. Stories that, under all the events and characterization, are essentially hopeful. Basically, in a Baen book you should know pretty well who the good guys and bad guys are, and the good guys need to win. Dark and difficult things may happen in a Baen book, but the whole story can't be dismal.
Note that manuscript mechanics -- spelling, grammar, and punctuation -- aren't on the list. That's because, as Toni puts it, "Story trumps all." So a good story (i.e., a good SF or F story) has a chance even if the manuscript isn't pristine. But you still need to proofread well and correct all the typos you can, because you don't want us to get distracted from the story you're trying to tell.
When I ran this list by Toni, she wrote,
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