I don't think the task switching in iOS is that bad actually, I'm happy with it and prefer it over Androids task switcher. But the inexcusable missing feature in iOS is multi window support. It's less important on the iphone since it's such a small screen, but on the iphone 5 it makes a tiny bit more sense. On the ipad it makes a ton of sense and iOS is lagging behind its competitors, Windows has it, and at least the Note has it as well and I'll bet Google is looking into it as a feature.
Why do you find a double tap of a physically button then a generic icon better then a single button press displaying a preview of where you left off in that app?
Android task switch is far superior in every way IMO.