"There will not only be stress tests for the new servers' coding and hardware configurations, but also tests for what will be the backbone of the game's new system, which is why we're being very cautious with development. Bear with us for just a while longer," says Naoki Yoshida in the latest producer's letter.
Yoshida also says that the dev team will likely keep the current version of FFXIV available for a time, even though no character data accumulated after November 11th will be ported to A Realm Reborn. The decision is based partly on player poll voting. Yoshida says he was "planning to keep the servers open so long as 30% of the players voted for it."