If I had to take a whack, it'd be things like the following:
- Interface can be confusing to some people. It isn't exactly clear that the tray is a most recently used list, and people assume it is a running app list. Especially people who expect to manage their background apps manually can find this backwards.
- WebOS has a fairly slick interface for switching between apps. Some people would like to see iOS follow the lead on this one, because a tray of icons is bland versus a screenshot of what you were doing.
- It's still possible to write a battery guzzler, thanks to the location services API. Trapster was bad because it initially forced you to kill the app in order to stop it from using your GPS (I don't know how/if they fixed it). TomTom is annoying because you need to clear the route in order to turn off the background GPS (or turn off the option in the settings). In the case of TomTom, people are expecting a big "Stop Navigation" button like other apps/devices, which TomTom buries in the Route Options > Clear Route menu item in comparison.
- There is no specific or realistic limit to the number of most recently used apps. This means manual cleanup is annoying.
- There are always those who want full control over their device and information at a glance/fingertips, as power users. These are the folks that jailbreak/etc to get lock screen information, SBSettings (which is something I'd like to see Apple implement), and so on. These are the type of folks that want more concrete control over multitasking (and what is background vs not) than Apple allows.
That said, I'm fairly surprised at how well the multitasking in iOS4 actually works. After seeing the failure that was Windows Mobile's memory management, trying to kill background apps when memory was low, but never really doing it right... to see iOS handle it correctly is definitely an improvement. I was personally skeptical after the announcement for this very reason. Apple's whole model hinges on the ability to kick out apps from RAM when the current app needs more. If that fails for any reason, the experience becomes degraded.