Good question. I've asked around, both on MacRumors and elsewhere, but I haven't heard a satisfactory answer, although it's pretty much agreed by all but the most rabid fanboys that the new UI is more sluggish and glitchy than the 5g iPod's. For most people it's not a deal-breaker, but it is for me so what the hell, I'll get a touch-screen device and still be plenty happy.
I can't believe the problem is with softwarethis IS Apple, after allbut instead the problem almost has to be hardware-based. That's a small form factor trying to squeeze a lot of processing power out of what has to be the smallest possible electronics. Something had to give when Apple decided to push the UI to the max, and it appears that occasional catches and glitches were deemed acceptable.
I realize that 90% of iPod users either don't notice the slow responses to user inputs on the newer iPods, or they overlook the problems because the iPod really is still a great product.
When I press a button on a small consumer device I want to know that I've pressed the button by seeing something happen with the UI, but with the 5.5g iPods, and now the 6g iPods, there are too many lags between input and response, and the occasional lags are unacceptable to me.
Not knocking the iPod per se and I'm not knocking Apple. I just believe the classic iPod O/S has outgrown the hardware, and it's time for me to move on to a music player that responds the way I believe a $200-300 device should.
Edit: Now that I think of it, I do think it kind of sucks that Apple has apparently sacrificed practical performace and went instead for eye-candy like Coverflow and art work and things I have absolutely no need for, even though most of the public may love it. Apple doesn't make the iPod for me, it makes the iPod for the general public, which, when I think about it, makes me happy I'm not happy with the newer iPod's, hehe.
