Interesting pov indeed. I consider myself that the drive noise interferes just in between songs (when it spins and loads the file onto the cache).
I do concur though with you hard disk, battery and current assessment. Unfortunately that idea of yours (and of mine) did not translate well into my ears when i listened back and forth to my 09 Classic and 2010 Touch: they sounded the same with AAC 320 Sheryl Crow files (Shure SE530 iems).
And i don't think the Touches mount exactly the same audio machinery than the iPhones. My iPhone 3GS and iPhone 4 HPO is a little better than the one of any iPod Touch i've come across. It kind of makes sense... if the iPod Touch features a slightly worse screen ,slightly cheaper buttons, slightly worse camera.. i guess something in the audio setup is slightly cheaper as well (Count the classic in here as well).
Once again this is just my 0.02
That's a pretty right sounding statement, I would agree. I don't think the output if affected all that much, either. It's mostly that the drive seeking 'noise' can be heard really well through earphones on hard disk players - the amp being affected might not be easy to hear. (I haven't had close enough sounding players to test.)
I read somewhere the audio hardware of the Touch 4G was the same as the 3GS - a definitive plus, because my 3GS is miles ahead of the Touch 2G I had before it. If that's not correct, though, consider me blushing. The rest of the Touch, like you said, being a little cheaper in build quality is something I can concur with - the Touch 4G is our house does have a nice, high-res screen, but the brightness is below the iPhone 4.
The Classic's biggest downside is, in my opinion however, the fact that Apple hasn't done anything with it for the last few years. There's tons of room for improvement (iOS without Touch per example) in that thing and Apple is, well, dumping it, which is a shame.