So then, here's my categories, feel free to add your own, and rank them in order of imptorance:
Price/GB - obviously the cheaper the better.
Reliability - I doubt it will die within 3 years, but I don't think I want another deathstar.
Speed - I may mod my case for an eSATA port, since USB 2.0 is actually worse than firewire 400 (I have both of these). However, it's all about backup, and ideally this is where all my torrantz would go.
Type - So there's the cheap premade ones, like the WD my book, or I can get my own WD Green drive and an enclosure, or I can grab one of those hot-swap n64-like docks.
Physical Size - This is my call, since having a slim external would be nice for travel, but I think I want a TB drive.
As for the enclosures: I currently have 2 Deathstars 160 GB in a Raid 0 and an aging Seagate Barracuda 320 GB. That means maybe I could get a better internal and use one of these as the external, but then I have a rather small external.
"Parliamentary inquiry, Mr. Chairman - do we have to call the Gentleman a gentleman if he's not one?"