That's correct. People could rip their CDs and also use MP3s they already had. The Kindle wouldn't even allow Amazon's own MobiPocket books that had Mobi DRM on them to be read! And, of course, neither the Kindle hardware nor software for platforms can read ePub.
You can get any content onto an iPod, but you can't take the DRM-protected content bought thru iTunes anywhere else, including streaming with Slimserver, etc. And people now presumably have a large collection of content that is managed the iTunes-way, with a significant subset that is DRM protected. Meaning you're locked to Apple's devices.