- the need to install something explicitly (separate from the app)
As widgets, apps provide them, you don't have to install them separately.
- they are completely invisible unless invoked by the user, thus preserving the simplicity iOS
As with widgets. You don't see them, until you start using them.
they have a natural and fixed size that helps preserve the visual aesthetic of the grid layout, without special efforts by the user
Actually, different sizes are nice, as widgets and Windows tiles have shown. Some like their calendar widget to be short, to just show the top 1 or 2 items. Others like their calendar widget to take up a complete screen, so that they can swipe to it.
I think the nice addition here is more how you create/invoke a widget and that it is restricted to one widget per app. But it's not all that different from Android widgets, WP tiles, or Blackberry's widgets.
They are Android widgets, but less discoverable. Actually they are not discoverable at all. I don't see how this preserves "iOS simplicity" to introduce a feature that is not simple and can only be known by word of mouth.
I would argue that iOS users also don't know that they can double tap the iphone home button twice to pull up the ability to kill an app without someone telling them that functionality exist or they accidentally find out. This widget concept would be no different.
- the need to install something explicitly (separate from the app)
- the need to name them "widgets" (to the user they're just a fancier icon), which reduces the apparent complexity to the user
- they are completely invisible unless invoked by the user, thus preserving the simplicity iOS
- they have a natural and fixed size that helps preserve the visual aesthetic of the grid layout, without special efforts by the user
...and probably more - that's just off the top of my head.
I don't know if they're a good idea, by the way. But it's not good to just dismiss them as "android widgets".