Mists wind-down was nothing compared to what we are seeing now. Warlords has pretty much been winding down almost since its release. Lackluster content has sent folks back to clear other facets of the game sooner than they would have previously.
Cross-realm zones only seem to have two specific limitations. Firstly, they will not be connected to anything more than three timezones apart, according to the hotfix they implemented shortly after it went live. Secondly, PvP and PvE servers will not connect to each other (we already knew that, but yeah). Outside of that, I have not noticed anything else that would stand out as a limitation to what servers you would see.
My only other guess as to who we see in cross-realm zones would be related to the datacenters. I have never bothered to keep track of who I see when I am out running around older content. If you have, by chance, you could compare that list with the datacenter listing: http://wow.gamepedia.com/Americas_regio ... datacenter