Spring-loaded Folders are the ones that would pop open if you need to drag a file across a large span of folders then they'd pop closed again at the end. Exactly what you thought they were. And they're VERY IMPORTANT. Apple NEED'S to bring them back. Remember, the features missing from X aren't missing because Apple doesn't WANT them. They're missing because they were never IN the OS to begin with. It's up to us to pick what we want back and tell Apple about it. Notice that most features that weren't in the OS when Steve left in 1985 aren't in it now.
Spring-loaded Folders
Tabbed folders
Windowshading
The Menubar Clock (Was missing at first in the Beta. Apple returned it after we complaigned)
Apple menu Aliases (They weren't introduced until System 7 changed the file system layout. Now they're gone again.)
HD Custom Icons (Were also missing in the Beta up til 10.1 and weren't possible in System <6 either.)
All the rest of the stuff is just NeXTStep features Steve's trying to push on us. While many of those features are cool, some aren't "Mac-like" and will only be changed if we tell Apple to change it.
Oh, and efoivx, I should have known it was DropDrawers. I Just noticed its icon in the menu in your screenshot. D'OH! Anyway, thanks for the link. I love them.
BTW, in DropDrawersX, is it possible to hide the applications icon in the Dock? I don't need it showing and would be much happier if it were hidden. It's also brightly colored and distracting.