They're "gating" the raid dungeon.
Basically, right now you can only fight the first 4 bosses, which are:
Lord Marrowgar - (who sounds like Richard Nixon, srsly)
Lady Death Whisper - (Female Lich... funky)
Deathbringer Saurfang - (High Overlord's Son. He likes cleaving stuff)
Gunship Battle - (You defeat, but don't kill, either HO Saurfang or Muradin Bronzebeard, depending on your faction.)
28 days later, they'll add the plague wing of the Raid which has:
Rotface
Festergut
Professor Putricide - (Gooood news everyone! Obvious Farnsworth reference)
After that, there will be another wait period of less than 28 days and they'll add:
Blood Queen Lana'thel
Her Blood Elf Lieutenants (3 of them)
Then lastly, they'll add:
Valithria Dreamwalker
Sindragosa
The Lich King
They said the 28 day delay for this first section was because they didn't want to pressure everyone over the holiday period.
It could be a few months until Arthas himself is killable. But he will be killable eventually.