Like the group finder it will have its downsides along with the convenience. When the group finder was added it did make it a lot easier to find groups but if you wanted quality groups you still did it the old fashioned way. With the cross realm pug groups they tended to be done at a mad rush but more importantly, they seemed to lose all social aspect. It was often like running with bots. And any interaction tended to be negative. Because it's so much easier to be rude to people from other servers you'll probably never have to play with again.
I can only imagine the kinda stuff that would go on in pvp areas that are cross server.

Seems like a great idea! Outside of the newbie zones, I'm shocked if I run across more than one or two players in each zone. And usually most of them will be 85s going for achievements! This seems like a great way to up the player population. I wonder if this will extend to Wintergrasp (and Tol Barad once it's outdated)? It's next to impossible to get some the achievements in WG these days without fudging stuff with the opposite faction (ie, getting them to stand naked on their mount and get blown up by cannons 20 times).