BANNED
Primeval Titan
Worldfire

UNBANNED
Kokusho, the Evening Star

BANNED AS COMMANDER
Kokusho, the Evening Star

Worldfire
This banning was largely expected. Though the card itself isn't overpowered, it does have unfortunate interactions with the format, namely that the Commander is available to be cast even after the spell has resolved, and our philosophy is to avoid cards like that. Since, outside of this one quirk, there aren't a lot of interesting applications to the card, we don't anticipate it'll be missed much. Unlike...

Primeval Titan
One of the concerns that we've had recently is the overrepresentation of heavy ramp strategies, to the point where it makes up a large proportion of the aggregate decks out there. While we think ramp should be good - this is battlecruiser Magic, after all - it's probably a little too prevalent and needs reining in a bit. With that in mind, we're banning the most egregious offender, Primeval Titan.

This decision won't be universally popular. Primeval Titan is dripping with awesomeness, and we ourselves are big fans of the card. But its ubiquity and effect on games couldn't be ignored and sad though we are to see it go, we think it will make for a more interesting and diverse format.

Kokusho
It's appropriate that Kokusho comes off at the same time as Prime Time goes on, as Kokusho was originally banned along the same lines. His presence had a similar warping effect on the format in the early days, with too many decks reusing the Dragon over and over (even if it didn't start in their deck!). However, in the intervening time, graveyard hate has become stronger and the overall level of creature power has risen to the point where we're comfortable - moreso after some testing - that it won't have the same impact.

He remains banned as a Commander because the mechanics of being a Commander allow him to circumvent the best method of dealing with him - the aforementioned graveyard hate. Getting him into exile as a creature is the end of it. As a Commander, it's license to start again.