Computer power users like composability, interoperability, "unix philosophy" / "do one thing well" -- whatever you want to call it -- because it's a powerful tool.
RodgerTheGreat summed it up really well:
> On a desktop, any user can invent new workflows. On an ipad, a user can only buy them wholesale. Users are at the mercy of the degree of forethought the app developers put into their designs.
Power users are frustrated because we like the iPad hardware and want to be able to use it, but Apple seems to have latched onto the idea that good tablet design requires them to sacrifice this idea. I and many others don't believe this is necessarily a requirement. Yes, the form factor is different, and you need to be careful with input methods, but good composability should still be possible, and could even be better than on a PC -- but Apple doesn't seem interested in actually trying. That's the criticism.