Having a laptop as my primary computer, and over 90 gigs of music on an external drive, I can definitely contribute to this thread.
The most important thing I've found when considering external hard drives is the selection of the enclosure with respect to the controller chip on the enclosure chipset. There are a great deal of chips out there that, while cheap, generally cause a great deal of headaches. Namely, I hold the Prolific PL-3507 and the Cypress-based chipsets in bad esteem. I've used enclosures utilizing these chips and I've had nothing but problems from them. Whatever enclosure you get, make sure it has an Oxford Semiconductor based chipset. They're faster, and a great deal more reliable.
Secondly, the hard drive you pick is important. I recommend Seagate drives because of hte 5 year warranty. They're readily available at very decent prices all over the internet.
Thirdly, the case construction. Don't bother with anything but an aluminum case that's fitting to the drive you have. For example, even with an aluminum case for a 5.25" drive that you fit a hard drive into, a great deal of heat will be trapped by airspace betewen the drive and the case. You want a case that fits the drive perfectly so heat is able to be dissipated by the case material. Plastic cases are just insulating boxes...avoid them.
Lastly, get a firewire-based enclosure. Firewire, though having lower overall maximum throughput, has much higher average throughput. Firewire data transfer speeds are not dependent on processor bandwidth in the way USB-based transfers are, so generally, they're much, much faster. If you dont have a firewire card, spend the 20 bucks and get one. If you have a tower, and wanna spend a little extra cash, get a firewire 800 enclosure and controller card. The speeds are ridiculous.
That's about it. Let me know if you have more questions. I currently have a dual OWC aluminum enclosure with 700 gigs of drives inside. It works very well, is fancooled, and has the Oxford 922 chipset. Im sold!