Rewriting

Writing is easy. Granted, there are some people to whom writing comes more naturally than others. There are some people who excel at dialogue, while others are better with descriptions.
But in general, it's a pretty simple process. You sit down in front of the computer (or notebook if you're old-school), and you write what comes to mind. Anyone can be a writer. What separates the women from the girlies (and the men from the boys!) is the ability to rewrite.
Rewriting is hard. Chances are that by the time you've spent 70,000+ words and hundreds of pages with your main characters, you've fallen a little bit in love with them. You have favorite scenes, like the one in which the hero tells the heroine his tragic life story in the most poetic prose you've ever written (so what if it's #extraneous to the plot? It sounds pretty!). You don't want to delete a word of your novel - why should you? Surely it's perfect as it is. And even if you know it's not perfect, you're probably so exhausted from the five-month-long writing binge it took to produce the thing that the idea of going back to rewrite it all makes you want to go to sleep for five more months.
So take a break. Set the novel in your drawer for a few weeks, go for walks in the sunlight (remember that?), eat lots of fast food, whatever. And a few weeks from now, when you're bored with your day job, take it out again and start reading. I guarantee you'll immediately pick up on things you want to change, even if it's just little stuff at first. Awkward sentences, grammatical mistakes, that sort of thing. Try to resist the urge to fix all of these - you've got to tackle the bigger issues first.
Are there any loose ends? Characters who disappeared halfway through the book, never to be heard from again? Are there so many characters that they all blend together into a mesh of one-dimensional cardboard people even you can't keep straight? Does the plot make sense, or did you forget to mention in the beginning that the hero was given an all-important magical sword which he uses to defeat his enemy at the end? Focus on the big stuff during your first read-through. Don't be afraid to delete stuff: useless characters, extraneous scenes, over-flowery prose. In Stephen King's On Writing, he says that the second draft is your first draft minus 10 percent.
If you have qualms about hitting backspace (like I do), just save your next draft as a different file (Best Book Ever Draft 1, Best Book Ever Draft 2, etc). That way, if you decide later on to add back a scene you deleted, you haven't lost it forever. I should mention here: despite the fact that I save every draft or alteration obsessively just in case of an event like this, I've never once added back a scene/character/flowery prose moment after I deleted it. If you can survive the pain of cutting out what may be your favorite lines in the book (or "murdering your darlings," as Stephen King puts it), you'll see how much better your novel becomes without them.
Last but not least, the thing that helps me rewrite more than anything else: reader feedback. It can be frightening to place your darling story into someone else's care, and ask them to tell you what's wrong with it. It can be even more daunting when your darling returns covered in red slashes. Just remember, your reader is criticizing the story, not your writing skills. Even the best writers in the world have to rewrite their stuff (unless he or she is one of those sci-fi/fantasy authors who is so famous that all of their books are 1,500 pages long, 500 of which describe every characters' outfit in minute detail).
Personally, I go through a miniature version of the Five Stages of Grief when I receive reader feedback. I start by reading through all of the comments, good and bad (the good ones help buoy you up when you hit the bad ones). First comes denial: "This opening chapter isn't an info-dump! It's a very necessary backstory; you need to know how my character was born, and what her favorite food was at age six. Nevermind that the plot doesn't kick in until she turns twenty..."
Then anger: "Why does my prose needs cleaned up? I don't very much quite understand how all of these lovely, sparklingly beneficent words could be superfluous!"
Bargaining: "Well what if I don't delete the 'entire' first three chapters? What if I just deleted two and rewrote the third one as a flashback later on, since the information we get in it becomes important toward the end?"
Depression: "Oh my god, everything I write is crap! No one will ever want to read this! This is the worst book in the world. I might as well burn all my revision notes now..."
And finally, acceptance: "You know, my beta readers are right. If I get rid of the first three chapters and jump right into the plot, it would hook the reader faster... And I can conflate these two characters into one, since they're so similar anyway, and... Say, rewrites might not be so bad after all."
The moral of the story? Rewriting is a long and painful process. You'll have to delete writing you love. But in the end, your novel will be a hundred times better for all of the work you put into it.

P.S. - And for those of you out there doing the proof-reading rather than being on the receiving end: don't hold back. If something sounds weird to you, or if a scene loses your attention, or if you just plain hate a character you know you're supposed to like, tell the writer! They might be upset at first, but it will make their story so much stronger.