What does the "experimental" DX11 support for WoW with Cata mean for those of us with DX10 and/or older graphics cards mean? Is Blizzard seriously considering a complete DX11 makeover? Also, for those that have played in DX11, are the graphics improved that much? Is it worthwhile for a graphics card upgrade to play WoW in DX11 mode?
No, Blizzard is probably adding it as a high-end graphical option. DX9 will definatly remain supported, and I'd be surprised if existing DX10 code was eliminated as well (though thats a possibility...)
WoW will continue to support DX9 - It's in Blizzard's interest to keep as many players as possible, which means keeping minimum requirements low but allowing for higher spec systems for those as have em.
As for DX11 vs DX10 - it depends a LOT on the game and how its implemented - mostly DX11 won't rock your world, but for some people it's enough of a change to be a must-have.