In Cataclysm, there were many hot-button death knight balance issues, but one of the biggest was Unholy Presence vs. Frost Presence. Frost Presence fell out of vogue very quickly in Cataclysm, since all DPSers found themselves running into a global cooldown wall pretty quickly. We were producing resources so fast, we needed the 1-second GCD provided by Unholy Presence to have any hope of spending them all.
This issue has come to the forefront even more in Mists of Pandaria beta testing. This is because a loss of runic power generating talents have put the dual wielding frost style at a disadvantage, enough that it needs the extra runic power regeneration from Frost Presence. Unfortunately, when it loses the 1-second GCD of Unholy Presence, it starts to stutter, with many testers using the dread descriptor "clunky" to describe the resulting rotation. Part of this could be trying to adjust going from Unholy Presence to Frost Presence, admittedly, but it feels like a problem.
If you want to see a comparison, switching to two-handed frost provides it. Thanks to the improved runic power generation on Might of the Frozen Wastes, two-handed frost can use Unholy Presence with ease, and its rotation plays nice and smoothly, very close to what it is on live.