Revising and editing - some pitfalls

When I started writing, I didn't realise how much time I'd spend having to rewrite what I'd first written.  There seem to be a number of reasons to revise what you've written, and here are the ones that come to mind:

1. you've come up with a brilliant idea which you're going to have to change a number of earlier passages to sew in.  This is usually, in my experience, during the middle of the first draft, and is a great feeling.  The problem occurs later, when you realise that what you've written in ignores the thread that you'd been on when you wrote it the first time round, and you need to spend even more time revising later on.
2. you realise that you need to tighten things up generally, so you need to pull out some passages.  The problem again occurs later, because the passages you wrote were more integral to the plot than you realised, so you have to re-re-edit.
3. you realise that you need to cut things down because your word count is too high.  You choose some unimportant paragraphs, which clearly have no major relevance to the plot.  See above (2) for exactly the same drawbacks.
4. you decide to improve characterisation, because a particular character was, at most, 1-dimensional.  You add more information about them which either contradicts what you wrote before, or changes what they would have done in later passages, requiring a much more major revision.
5. you discover that what you wrote reads as if it were rubbish, and it contradicts or destroys a major plot point, but when you look at it in detail, you just realise that you wrote it in such a way that it was very unclear, and a little tweaking will allow your reader to follow what you meant to say much more clearly.
6. you discover that what you wrote reads as if it were rubbish, and you're entirely correct.  The problem here is that you really did write rubbish, and it may have been compounded by any of the previous points.  This is a big one, and may require you to start again.  But it's only words, right, and at least you realised before you sent it to an agent.  You did realise, right?  

So, revision is vastly important, but there's a danger that when you come back to a piece of writing having let it rest for a month or more - as everybody says you should - that you're out of the zone of tight plotting that you were in when you wrote it, or at least believed you were in, and that it's therefore very easy indeed to mess things up royally as you revise.

But that doesn't mean it doesn't have to be done.

I didn't mention a final mistake that you might have to edit out.  You might have made some insane decision when you started writing the piece that looked, at the time, as if were bound to win you a major literary prize, or maybe a Nobel, but which, in retrospect, just shows that you should ensure that you're entirely sober when you start plotting.  I should point out that nobody would ever do something like this - it's the equivalent of writing your narrative from the point of view of a baby elephant - but if you were to do anything quite so crazy, it might be time to think about some major changes.