It has a nice balance of features and elegance -- does a lot of useful things without doing a bunch of stuff you don't want it to do (MS Word, I'm looking at you). The creator is very approachable and receptive to feedback, while at the same time having a very clear vision of what he wants Scrivener to do and to be. It won't be bloated just to be able to hype another feature, and bells and whistles won't be added in a haphazard, semi-broken way. That's quite refreshing for a piece of software! (He's also firm on keeping it Mac-only, which I understand and admire but is a shame for PC users.)

I also like that it supports various different styles of working (I find its infinitely rearrangeable binder is even more useful when you're not necessarily writing in order, but also want to see at a glance how well the pieces are fitting together). The research area is a nice place to collect all my notes -- I used to keep a wiki but realized I was using it mostly as a collection of text files and not using the interlinkedness of a wiki to its full advantage, so this fills that need for me quite well. I don't take full advantage of a lot of the features (like filling in metadata when I'm starting with an almost complete draft), but it's great to have them there, once you do have a reason to use them.

One thing I'd add to what you've already mentioned above: there's a "snapshot" function that allows you to save a backup of a scrivening before editing it -- it's a nice safety net (for me) to get over the anxiety of messing with something for fear that I'll make it worse and forget how it originally was. Besides, it's kind of neat to have a history of the way the draft has evolved.

It's great for longer projects like a novel, but I've also used it to collect shorter pieces and works-in-progress in a way that's easy to browse or search -- with all the same ability to tag it with keywords or notes, or create new revisions. I still use a plain text editor to do a lot of writing, especially first drafts, but I love Scrivener for the stages after the initial outpouring of words.

--ich sage nicht, was ich will, sondern was die Sprache will--