I am looking for an IOS command that would show me the content of specific packets (such as in Ethereal, but of course, less sophisticated).
Is there something more the DEBUG IP PACKET DETAIL (associated to an ACL) ?
You need to add the hidden 'dump' keyword to your debug.
debug ip packet dump
It shows the packets in Hex (only packets that are process switched) You will have to decode the hex yourself or use wireshark text-to-capture.
Hope that helps.