By far the biggest pain point about Amirdrassil is that it's not on Kalimdor. I think they'll be willing to wait if it's planted elsewhere and after the expansion.
The fact that its their home though makes me think that either there are multiple zones in the Dream area, one of which is the tree (possible), or the Nelf capital will be open for both factions (people would shit). Because otherwise a Horde players can't access the area.
I think it's the former because all the file stuff is the DREAM and not the tree itself. Regardless of the cinematic. Also the warglaive icon looked like generic dream greenery and not the blue associated with Amirdrassil.