I think Cloudpost might have been unnecessary - the Pro Tour had Cloudpost decks with less than 50% against the field, and less than 50% against every other major deck in the format.
That said, if they ban Preordain and Ponder and Rite to cut combo out of the format, it's entirely possible that Cloudpost would beat everything-but-Zoo.
I don't think Green Zenith was a BAD ban, but I don't like when they ban for ubiquity, especially in a single colour[/c]. Yes, every green deck was (and should have been) playing 4 Green Sun's Zenith. But it wasn't so powerful that your deck HAD to run Green for it, which I think is quite a significant factor.
I think Mental Misstep's ban in Legacy was a mistake; the format wasn't becoming less diverse because of that card; it was because Legacy has been full of terrible decks for YEARS and now the format is getting official recognition, the quality of the deck is improving and the bad lists are being pushed out.
Before Misstep, the format was basically Control Jank and Combo Jank v Merfolk with Merfolk winning all the time. I think removing Misstep causes more problems than it fixes, to be honest. And again, it was a ban-for-ubiquity, not really a ban-for-format-ruining.