Can anyone imagine why, when I drag a wmv file over to my local drive from my file server, and play it from the local drive, the file transfer occurs quickly, and Windows Media Player plays it fine, but if I try to play it directly from the server, it runs like molasses?

Put a network sniffer (Wireshark? Whatever Ethereal turned into) on it.

I suspect someone wrote some really, really Stupid file access logic (probably for Digital Restrictions Management) that gets cached nicely on a local disk and causes horrible network traffic.

Well, it used to work fine with my previous server. This one is using Fedora Core 7 and it has SATA drives (FWIW). I only just got Samba working on it. Anyway, I doubt if it's WMP, because that hasn't changed since I used to do it before the server upgrade. But it makes no sense that I can drag a multi-meg wmv file from one machine to the other in just a few seconds, but WMP can't handle the data transfer when it's actually playing it across the network.

Some WMV files store information at the end of the file. Seeking to the end of the file might take a lot longer via a network share.

It is called Wireshark (yum install wireshark).
I have to say, I do not know anything about Samba.

I've noticed that Microsoft File Sharing has real problems with lots of small transfers. Once at a company I worked for they were copying a server's data directory which contained a bunch of small files. The file transfer slowed to a crawl, just a few kb per second. I went over, and while the copy was still under way zipped up the files and copied it to the destination. They had been copying the files for hours - I finished the job in 10 minutes, recopying the entire directory.

So, when you copy the file it is one very large read. When you play the movie, it is lots of small reads.

(I've also had problems with network card's timing being messed up causing LOTS of collisions, but it doesn't seem to be the problem here.)