As for the rotation, who knows -- clearly the earth once spun much more rapidly, but if that was caused by the same glancing impact that created the Moon, then alt-Earth might have been much different.

A big issue, though, is plate tectonics. Our own Nick Hoffman, among others, has suggested that plate tectonics on Earth is only possible because so much of the light crustal material was removed and tossed into orbit. Otherwise, we'd have a much thicker lithosphere, too thick to subduct. In this view, Venus is the alt-Earth, at least in terms of large-scale geologic style. I don't think everyone would agree with that explanation, and I'm not sure Nick himself would characterize it as more than a good hypothesis at this point (Nick?).

In any case, I do think explaining why given two bodies with basically similar bulk properties, Earth has plate tectonics and Venus has whatever it is that venus has is a very important question. I'd like to see us giving more priority to Venus for that reason, though I'm not sure if there's a good way to get traction on the problem short of deploying a long-lived seismic network on the surface of Venus -- no easy feat.