Your mileage will vary ... it all depends on how many people are going to be banging on the system at any given time. The Pi could support any one of the things you've listed with varying numbers of users depending on the particular service you're considering.
Lightweight web serving, not a problem. I wouldn't try to run an e-commerce site for anything more popular than used Popsicle sticks, though
It should have no problem serving up torrent (seed) files, if the Pi is only one of many nodes serving up the actual content chunks (and you may not want to even have your Pi loaded down with that - your Pi can be the conductor, but let others provide the orchestra).
Acting as an SCM server should not a problem as long as you're not trying to support more than a few people at a time by serving up all 30,000+ packages in Raspbian (or maybe that's the file count - "In all the excitement, I lost track, was it 30,000 or was it 40,000? Do ya feel lucky today, punk? Well, do ya?"
).
Supporting an eBook catalog would depend again on how many downloads, of what sizes, you're trying to serve simultaneously.
A media NAS is going to be OK if all you're doing is watching one movie or listening to one song at a time. Serving up multiple movie and music files to more than one user is going to grind things to a stuttering halt, even if they're just on the LAN.
Running any random combination of all of these services simultaneously will most likely bring the Pi to its knees, bees' or otherwise, especially with more than one user.
The Pi is really a graphics workstation in sheepish clothing, not a server in most senses, even if anything without a monitor and keyboard is considered by some to be a server these days. It should be noted that the Pi may not be the bottleneck - your Internet connection will most likely be the slacker among all resources (well, the 256 MBs of RAM comes in at an extremely close second, if not a twin for first). You can support 1,000 network users if the bandwidth is only a few Mb/s and each user can only extract a few hundred bits of data per second.
As always, Your Mileage May Vary, Do Not Spindle, Fold, or Mutilate, and Objects in Mirror Are Closer Than They Appear
