I think a lot of people have a very unrealistic concept of what editing actually is. iMovie is a fully capable program if you know to deal with it's limited feature set.
An editing application isn't supposed to have a ton of bells and whistles. Take a look at a Hollywood movie sometime and really look at what it took to put it together in terms of editing. Basically you have straight shots pieced together with 95% of the time straight cuts. Every now and then you have a cross dissolve or fade to black. Even documentaries rarely move beyond that. The ironic thing about iMovie is if you actually approached a professional project with it the way a professional project is edited then yes you could totally use it. It is when you start to add the cheesy stuff that you only find in low grade productions that iMovie starts to suffer. Even then it does have some of that stuff built in.
About the only really lacking thing in iMovie is the film source limits although I think some of that is being addressed. The latest iMovie even supports native 24p AVCHD footage. Right now the way iMovie is you could totally shoot a feature film on a DSLR at 24p, import into iMovie and cut together your entire movie.
If you want to dive into more advanced animation you can always buy Motion for $50.00 and render segments to use in iMovie. I work for a professional production company and our animators do all of this and render video clips that the editor cuts together with the raw footage. No reason the same couldn't be done with Motion and iMovie.
If you really must have multiple tracks then by all means by FCPX but again very few true professional productions actually need that sort of thing. To a pro anything above a single track is considered animation or motion graphics and is better off treated that way.
If you want better encoding tools then but Compressor for another $50.00. There you fill in two of the major gaps in iMovie with $100.00 of software.
I agree that the key in good editing is putting together the right shots at the right time for the right time. No big features needed creatively, but I could imagine that with fcp x you could control dissolves much better.
In the technical division, this might be different, especially in the sound department. Somebody mentioned sync and here's where fcp x would be great.
I have the impression that where post production really counts is after the film leaves the editing application (I mean the technical work, the look. The editor is so important, he makes or breaks the movie).
I mean the real high end work done in post production like getting the color consistent, getting the light consistent - the look of the movie.
There's a great clip on Adobe's website/ After Effects about David Fincher and his editor talking about how they styled "The Social Network". How they cleaned up the images, removed a camera, for example, and then even split frames to get the timing right for the dialogue.
...wondering what the output quality of iMovie is, compared to fcp x.
And if you really want to edit film if it pays to save the money on fcp x.