UI elements stay the same size so are as easy to hit as before, but because the change in pixels isn’t exactly double the art needs to be interpolated and so edges blur and you lose the crispness. For fast moving games or videos it won’t matter. For anything with line work, grids, borders, etc. it will matter a great deal. Stretched apps don’t look as good.
What do you do with a one-pixel-wide black line on a white background when the resolution is multiplied by 1.5? You can’t draw a half pixel line, so to keep the UI the same physical size, the set of pixels next to the black line has to become grey.
In other words, if Apple increases the iPad’s resolution without doubling it, iOS is going to get ugly.