The funny thing is, when you go back and read it six months or a year later, you can't tell the difference between the bits you slogged through, cursing every word, and the bits that came out as if Odin himself was feeding you the lines.
There are one or two whole books that I've written in which there is just so much struggle to roll the boulder uphill that I develop a reflexive cringe response to the text itself. All of the parts that I actually like are the parts that were not such bloody-minded struggle, and I am convinced that the book is an abject failure because there were not enough of those parts to support a book. It takes me about two to three years to forget the experience of actually writing the book, so it takes me about that long to be able to read and parse the words that are actually there, as opposed to the words that I'm terrified are there. Which is another way of saying it's hard for me to reread anything I write until I've forgotten the writing of it.
(Yes, this makes revising and editing a big strain, because I am frequently changing words that I hate into, oddly enough, more words that I hate, with no certainty that the changes actually do anything useful. But waiting 2-3 years to revise a book would be a bit problematic.)
But matociquala is right. When I've forgotten the struggle to put one word in front of the other in a way that approximates a native English speaker, I can't actually tell which words were the ones that I struggled for hours on, and which were the words that came easily and naturally, as if they were an act of grace.
This implies strongly that there isn't, in the end, a difference. And for the people who weren't mired in the writing, this is probably true. Although it can be true in the "these are all good" or in the "these are all dreadful" way; the point is, the words that are a total, fun-sucking slog do not, in fact, stand out from the words that weren't, regardless of how one feels about the book.
The difference while writing is this: the words that come naturally and easily are words that are easier to trust. They do not automatically feel like garbage. They do not automatically feel like they're full of fail. It is easier to find the shiny bits and point at them and feel, somehow, that we've done something good. There are so many little insecurities and struggles with the ever-smarter Internal Editor while writing any book, that anything that somehow makes the words seem decent and worthwhile makes the book easier to write. People who feel that they need the inspiration or they write crap might not, in fact, be writing crap when they are not inspired -- but their perception of it counts.
Because when it's a slog, it is definitely Not Fun.
(Yes, I am in the middle-of-the-book. Why do you ask?)
ETA: This, I realize, should come with the usual disclaimer: This is me, talking about my process, and my finished books. Other people's process will probably differ widely.