-iTunes is needed to move files on and off the device
-Proprietary 30-pin dock connector
-prone to scratches
The device has a decent heft to it, to give it a premium feel (although weight should not be what one uses to determine quality but we've associated heft with quality nowadays). The back unfortunately is prone to scratches due to it being a highly polished metal. While it looks gorgeous this is definitely something you would want a case for. The screen can be scratched a bit easily due to being "regular" glass instead of something like alkali-aluminosilicate glass (Gorilla Glass) or some of the other more scratch and shatter resistant variants.
Measurements from the 3.5mm jack are great from the tests I've seen. From my listening experience it has a nice black background and most headphones sound clean through them.
One large complaint is that you can't expand the storage. 8GB is extremely limiting. Your best solution would be to use an application to stream music whether it be a music streaming service or a cloud storage service.
The proprietary connector is annoying if you lose the cable or happen to forget it when you need it but it's where a lot of Apple's extra profits come from.
My largest complaint is the fact that you must use iTunes on your computer if you ever want to move anything from or to the iPod using the computer. iTunes on Windows has generally been a bit of a mess and it's not exactly pleasant to be tied down to a piece of software for this device to interact with your computer. At the same time it makes organizing media more "interactive" in a sense. For the majority of consumers it's more intuitive to be able to drag the album or song onto the device instead of using a filesystem to drag and drop files to specific folders like video, music, or photos which you can accidentally mix up if you're not careful.