I almost never agree with his politics, but Charlie Stross
has a whole heck of a lot of interesting things to say about writing novels (and he just finished one, too).
The next thing to understand is that you never have as many ideas as you think you do when you start out writing a novel. Your novel absorbs material like crazy; you throw loads of stuff at it, stuff you're used to leaving out of your short stories because there simply isn't room, and the novel just sits there in a corner and belches, then says "think you're tough, eh? Gimme another sub-plot! And a recomplication on the side!"
I think the one novel and several drafts of novels I've worked on suffer from this. Not enough going on. They're far too linear.
Plots can come in bits'n'pieces, but usually follow a story arc. The novel begins with scene-setting, then something disrupts the status quo. There's a long rise in tension as the disruption recomplicates, adding complexity and making our protagonists Do Things. Eventually we hit a resolution point at which things Change -- the climax. Finally we have a look at the new status quo (optional, if you're good enough to imply what it will look like to the readers before you get there).
And here's an interesting suggestion:
Don't get silly and try to write a multi-threaded novel straight off, you'll tie your own shoelaces together and trip over them. If you must do multithreaded, a better way to do it is to write a novella -- say, 30,000 words long -- and then write a second novella of the same length showing the same story from a different angle. Then intercut them chapter by chapter, like chunks of salami. The trick here is to find a story that has enough different angles to be worth looking at repeatedly.
Anyway, if you're interested in the novel-writing process, and the view of a writer who just drafted 90K words of one, go have a look. Interesting stuff.