Okay, so, I said I might be putting down Fountains of Paradise every now and then to read something else. I didn't. It's the taste I wanted, and the pacing worked for me. And, at one point, as the main character was trying to figure out what he could possibly do to get out of a jam, for the first time when reading sf, I figured it out just a beat before the author told me.
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Too much earlier, you're just marking time (though as I did when reading...er, what was it? Something recent...you can sometimes notice the solution, then get distracted and forget about it for a hundred pages, only to pick it up right before its brought into text. Oh, right -- _Un Lun Dun_). But if you -never- see it coming it isn't as satisfying, and maybe the writer isn't leaving enough clues.
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It can also make it feel like the characters are smarter than you.
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Similarly, not -all- reveals need to be guessable -- but it's nice if most of them are; truly unguessable reveals tend not to be as satisfying, as they're insufficiently prepared. Of course, Sherlock Holmes is smarter than the reader -- but then, he cheats, not actually giving them more than a small fraction of the clues he collects.