In one of the many interviews lately, it was stated that none of the raid bosses had been designed yet. While the Naxx bosses will obviously be quite similar in design to the previous version, from a development standpoint the main benefit there is that the artwork and dungeon design is being reused; that's what takes a lot of man-hours, when making entirely new content. The boss design and tweaking will still need a lot of work, to get balanced to a usable standpoint.
Of course, having both 10 and 25 man versions of each dungeon will add some extra work as well, although certainly it's a great bang for the development buck, when it comes to giving the players more to do.
It's nice to see the official thinking for the expansion is a release date of this year; hopefully it won't need to be pushed back like TBC. As for AoC or Warhammer, there's certainly been no reason to believe in any pressure on Blizzard regarding those games. Everybody made such a brouhaha before Vanguard was released, and how Blizzard needed to "respond", and it came and went and sank like a rock. Even if both AoC and Warhammer do well, they are minor blips on the screen, compared to the overall numbers in WoW. Whenever WotLK is released, you can safely bet it will
sell more copies in the first month than both AoC and Warhmmer combined will sell, during the first year (or possibly even the entire lifetime) of their respective releases.