For sake of argument (and not for sake of flamewars) I'll assume the opposite since I think there is a valid argument both ways.

I'm not sure I would hold the OS X finder up as an example of good design:

Complaints about the finder are numerous and common, there are more finder replacements than just about any other system utility. A google search for "OS X finder" reveals that in the top 10 results (excluding images), #2, #3, #5, #6, #7, #8 and #10 are all either complaints about the finder, replacements for the finder, or commentary on complaints about the finder.

The titles are things like "How to Make the Max OS X Finder Suck Less" as if it's a given that everybody understands it sucks. "n replacements for the Finder". It's even one of those rare topics that Gruber can find it in himself to complain about Apple. http://daringfireball.net/2002/11/that_finder_thing

Also, once I got over the loss of the investment of brain space to memorize every possible menu item in the MS-Office suite, I've actually found the Ribbon interface to be very enjoyable to use, just as fast for things I know about, and many times faster for things I don't because it makes them easily discoverable.

But let's assume for a second I step up to each computer with no idea how to use either (but am familiar with very basic metaphors like a hierarchical file structure).

Looking at the OS X finder, I have virtually no idea what any of the buttons do. Left to right:

arrows going both directions, one is grayed out. Knowing that files can be organized hierarchically, do those buttons let me jump between siblings?

4 pictographs of no discernible relation to anything to do with file management: e.g. does the second one open up a text editor? the fourth one may have something to do with audio volume or screen brightness

there's an eye, does that control my camera? open up a file? only view a file?

a gear/light flash/different sort of eye glyph from the one before. if it's a gear maybe this lets me adjust some sort of setting

at the interface itself:

what is a device vs. a place? what is the relationship to each other? is a place on a device? or is a place something different?

Why do I have two columns of what look like folders? are they related to one another? if the rightmost column is supposed to be what's inside of the selected library folder in the second column, does that imply that I can only go one folder deep?

then at the bottom I have something that looks like a hard drive (but I wouldn't know that having never seen one), a folder called 'users' (why are there more than one! it's my computer, I'm the only one!), a house and then finally another folder.

I'm left with more outstanding questions for what's not on the window. How do I create folders? How do I delete things? How do I rename something? Provided I can figure out that this is a representation of the hierarachical file structure, is there a way to copy or move files between different parts of the file system? How do I add or remove "places"? etc.

The principle problems are that the finder is simultaneously underpowered.
It also hi-lights something that's sadly becoming more and more common in Apple's interfaces, a lack of discoverability. Sure, the disconnected toolbar up at the top of the screen (or perhaps on a different monitor) may answer some of my questions, and I could hover for tool tips perhaps, but realizing that those things are even connected is a serious interface problem.

The new explorer isn't without it's faults:

why is there a "File" menu when I'm clearly looking at files, is that something I push to "file" away something?

it's got the weird left right arrows and even a weirder up arrow

why is there a manage meny? aren't I managing my files right now?

But for the most part it achieves discoverability, while also having (or at least doing a better job of revealing) power.

Oh! So you push the button clearly labeled "move-to" to move a file to someplace!

Oh! There's a button called "open" with a picture of the MS-Word logo, that must open that file in Word.

etc. etc.

"Also, refresh accounts for > 6% of Explorer commands. Why don't they just make Explorer refresh automatically and get rid of the need for the command?"

It does, just sometimes not fast enough for most users. It certainly works better on OS X.