Excessive heat will shorten the hard drive life, but otherwise it is quite a normal condition for them to run warm - I recall a recent study by Google (they obviously know quite of bit about running thousands and thousands of hard drives continuously), and the end result was that it was actually better for reliability to run them a little warm than cooled down to room temp or less. Some time back somebody posted here pictures of the Beomedia 1 insides, and the thermal design seemed quite good to me (unlike many regular PC enclosures). I don't think the hard drive will be too hot in there.
Thermal cycling due to turning on and off will be a stress to both electronics and mechanics. From my work, I know that before turning off a server that has been running for years uninterrupted, it is a good idea to take a complete backup just prior to shutdown... there's a good chance that the hard drive (or the power supply) will not start again after it has cooled down.
On the other hand, at home I have a HDR of sorts (a Linux machine running VDR) and I have configured it to shut down completely when unused, and the next scheduled recording is more than half an hour away (due to noise and the electric bill). No problems in almost four years since I built it.
Hard drive reliability is a controversial subject in general talk because it is all about statistics. Almost everybody has experienced one or two examples either way, but they won't tell anything about the big picture - any given hard drive can fail any given moment no matter how you treat it. The only way an individual can fight the statistics is to take backups 