1: Good question. I've found the hard drives I use to be pretty cool. Yes there is no fan, so computers have the potential to have cooler temps for the drives, but my computer puts out some real heat, and even with 2 120mm fans for inake, I've found the drives inside it to feel at least as warm to the touch. Not exactly a scientific measurement, though. Alot of people are adamant that heat is not detrimental to hard drive life, at least the common amount of heat in computer towers.
Also, most of the drive case surface gets constant exposure to air. The entire platter section is outside of the BlacX case. The part of the drive enclosed is mostly just all the connector junk. From memory, I'd say maybe an inch or so of the drive is inside. Its locked until you release it, so you dont have to worry about it falling out.
2: The BlacX itself makes no noise, but the noise from the HDs is going to depend on what make/model you have. I use these, and they only really make a spinning noise when being accessed from being idle. Its an odd noise, something I dont hear from my computer, because the case blocks it. It only lasts for about 1/4 of a second. Its not that loud though. I only notice it 'cause I'm sitting right next to it. In a HT setting, it would be very, very faint, if audible at all.
3: No not at all. Its not my OS drive though. Just audio. I've been using USB drives for audio/video/data for a while and they seem to work just as well as internal sata as far as accessing the data. Writing to and from is not as fast though. But nothing significant.