So Mystic Worlds wants more horizontal changes. That doesn't necessarily mean midlevel content (although a lot of her suggestions mean more things to do for midlevel players). Instead it means that Blizzard should take a breath, and set their teams on making the existing game experience more fulfilling. Things like guild halls, single player dungeons, more professions (Inscription will probably provide new items 1-70, but Woodworking has been necessary for a long time), and little touches-- more live events, gambling minigames (that all players can do), and fun little additions that all players can get involved in, not just those who have made it to 70.
Now, there has to be some part of Blizzard that is pushing for this stuff-- it's not quite as simple as casual vs. raiders in this case. But Blizzard seems to think that they're behind on high end content, and that they're rushing to catch up to players who are hungry for more of it. I don't think either perception is true. If anyone is hungry for new things to do, it's players who have more than three alts, and yet a main that hasn't yet reached 70. I'm more than willing to trade one of the three or four 25 man instances planned in WotLK for two or three of these "horizontal changes," and I'd guess most other players are, too.