While I'm all for cross-faction guilds, I don't see how this will work on a larger scale without cross-realm guilds. People are already fractured between factions and most players create their Horde/Alliance characters on realms with a large population of that faction. Even if I would like to join up with my friends I made through a community, chances are very high that we are from about 5 different servers and hardly anyone wants to pay for a server transfer just to be in a guild with friends.
I don't understand how so many people are upset about cross faction happening. Have you all not been playing the game? The cross faction war is over and has been over. Both sides are working together now because there are much larger threats out there who want all of azeroth or to destroy all of it. Both factions have come to a ceasefire and have a peace treaty at the moment. Our faction leaders have been working together since Pandaria. If you think it's a step in the wrong direction then you haven't even been paying attention to anything going on. Classic and RP servers still will let you be digitally racist don't worry.