Cross-everything clarification

I know Blizz has made several changes to the restrictions on how players from different factions and realms can interact. But I haven’t kept up with the finer details so I have a few questions…

What, if any, are the restrictions on guilds? Can they be cross faction or cross realm? If so, how does this affect server first raid races?

I know that raid groups above LFR can now be cross realm. What about cross faction?

What about casual grouping for questing and dungeons? Can those be cross faction?

I guess I’m just wondering what barriers aside from regional (NA/EU) still exist for grouping.

Currently, guilds can be cross faction, but not cross server. There are some limitations if you’re in another faction’s guild, like some of the faction based guild rewards. (Also, I’m pretty sure that the guild’s faction is set by the faction of the original GM, and you can’t pass GM to someone of the other faction.)

For the next expansion, The War Within, cross-realm guilds are one of the listed features. As for server first races, it means they probably won’t really exist any more. It will just be region based.

Cross-faction groups only work in instances (for the most part.) Out in the world, players of the opposite faction are still hostile to you, even if you are grouped. As mob tags are now faction-neutral, you can still work together, but you won’t be able to heal them and such.