Proper mastery of the flashback. Well done flashbacks can really enhance a story. Poorly done, they can derail the story entirely and make the writer look like he/she has no ability to plan, plot, or properly convey information.
I've written stories (though not novels) just to practice writing action sequences. I used to be very, very bad at them. Now I'm merely nervous about them. I'm not sure I've mastered them, but I know they rarely make me cringe anymore when I re-read them. They need proper pacing, enough description to give you an idea of what's going on but not so much description that it bogs things down. You need fighters to look competent, non-fighters to look bad but not laughable, you need to properly use technical lingo but not overuse it... I find them very difficult.
The same's true of sex scenes, btw.
Fast enough but not too fast, experience levels correct for each character, use the proper tools at the proper times with the proper language... Sex scenes are action sequences, I say.
You might try writing in a plot point that requires technical knowledge, to practice researching a topic and then conveying the right amount of info about the subject. Like, if archaeology is a big point in a book, an author has to research archaeology, then convey to the audience enough so they know what's going on, a smidge more so they learn a little bit, but not so much that it comes off like a how-to manual. I don't think David Weber has figured this out yet, and he's on his 5 billionth book. He gets going on weapons construction and it becomes a technical dissertation.
