I have a concern about fast app switching. I updated Wonder Reader just to confirm, but right now, this is the scenario in a nutshell: You load up an app (takes some time the first time), you have to visit another app for some reason, so you hit home (or follow a toast) and load up another app. The problem in NoDo is that, when you want to go back to your first app, you have to load the app again. Mango's solution for this is to press/hold the back button. You get a list of five or so apps, then you can select your previous app and you won't have to load it again. So far, so good, right?
Here's the problem I see: If you don't use that (hidden and non-obvious) method of holding the back button, the way you'd go back to your original app is to hit home, then tap the icon for the app you were working with previously. Unfortunately, doing that will cause the app to load from scratch again, even if there's a "version" of that app fired up and ready to go in the app switcher view!
In Android and iOS, you have the equivalent back button app switching method (double click home in iOS, hold the home button in Android). But you ALSO have the ability to go home/app launcher, then open the app again, and it will load up instantly (provided it was one of your recently used apps).
This is WAY BETTER than the mango solution, because it is how MOST people switch between apps, and is way more natural. I suppose we can train ourselves to always hold the back button instinctively, but most of us are going to unlock our phones and hit the Facebook app icon and wait for it to load. Most users won't unlock and immediately hold the back button, remembering that they used the FB app recently and can get back into it much more quickly that way.
Many people have compared the app switcher view in Windows Phone Mango to WebOS, and while they are similar, here's the key difference: in WebOS, that app switcher view IS the home screen, while in Windows Phone Mango, it's a hidden functionality that, if you don't utilize, will have you waiting for your apps to load even though there's an instance already dehydrated and waiting to be resumed.