I'll see if I can clear things up a bit for you.
+have set preferences in iTunes to set media folder to D:/Other hoping it would put everything iTunes data-related there+
That itunes preference is where itunes will put the
content, not the
database.
And it only affects FUTURE additions.
+but find that iTunes still puts its library in my documents (or Win 7 equivalent)/(My) Music, which I don't want it to.+
This means that at some point, you opened itunes without the D: drive being available, so itunes reset back to defaults.
+Also, when I imported my music collection (again with Edit/Preferences/Advanced media folder set to D:/Other), iTunes put (moved? looks like--copied? maybe) some of my music folders to D:/Other/Music. Why did it do this and how do I stop it?+
You can't stop it. That's how itunes is designed. Have a look at this post by turingtest2 on how itunes deals with folders
http://discussions.info.apple.com/thread.jspa?messageID=11369702�
My advice is to put your ITL file in the Other folder. DO a shift-start of ituens and choose that ITL file. And never, EVER start itunes without D: being available or it will reset back to C: