When looking at an enclosure check a couple things:
1) *Chipset - This is crucial - Cypress for USB2 and Oxford for FireWire, two best chipsets for their respective connections hands down. This will ensure that you wont corrupt your hard drive, prolific chipsets have been known to do that.
2) Fan - If your hard drive gets very hot, like a 200GB that I have (which burns up unless air is moving over it) then I suggest getting an enclosure with a fan. On the other hand, I have another 200GB thats cool to the touch, so its in an enclosure that has no fan. Either way, having a fan would probably increase the life span of the drive, but its up to you whether you want to deal with the noise.
3) Aesthetics - Its got to look nice where ever its going to be and not be an eyesore to you
Anyway the enclosure I got has a Cypress chipset, has a fan ( but minimal ventilation holes (could be modded, but the drive remains cool so I have no plans to mod it), and looks sleek.
I just ordered another AZIO enclosure and although it doesn't have a fan it has vents on the front an the back and it is all aluminum.