The advent of Real ID, allowing me to group with these disparate friends, has made my life in game a lot smoother overall. Why, just last night I convinced Anne over on CC to log on an alt long enough for me to go inside Ruins of Ahn'Qiraj and punch everything in the face. Just because. I had no reason; there were bugs with unpunched faces, and it was bothering me.
Recently, I've noticed a spate of discussion on low-pop realms. Low-population realms have been one of the most enduring problems World of Warcraft has had in its years of operation, so much so that recently it's been announced that Mists of Pandaria will include a new feature allowing certain zones to exist across several realms for ease of grouping and questing. The cross-realm zone concept is as much to help with the shortage of low-level characters in general (most of a realm's population is often at or near the level cap at any given time) as it is to help low-pop realms, but it's still a step in the right direction for them.
With Real ID and now Battletags allowing for cross-realm raiding and the implementation of entire zones across several realms, I find myself wondering if the future of World of Warcraft will see a radical shift in how we think of the realm and how it is used. We already can randomly group with players on many realms for dungeons, Battlegrounds and the Raid Finder. We can group across realms with our friends for dungeons, PvP or raiding, as well. We're about to be able to meet players from other realms as we level. What's next?