I think their main complain is about the age old file system, but that's the ancient way of doing things. It's understandable because people are used to it since the first OS was built. Other things are just minors.
You can't deny the benefits of a filesystem. The main reason is organization. Say I am working on a project. I have 50 PDFs, 5 Word documents, 1 PPT, and 10 photos that are related to that project. The iOS way would require all those files to be within their various programs with no way to group them together.
If there was a way to tag files and link it to a global search, it would be way more powerful. But, that's not present and my point remains.