I can kindof understand why you wouldn't want to be able to cross-realm heroic mode, since there are the world first feats of strength and whatnot, but since there really is no big prestige awards for normals, why not enable cross-realm grouping?
It would have seriously helped my group last night when we unexpectedly lost a healer with an hour to go for our raid time last night if I could have utilized my entire list of friends instead of just those on my server, and I play on a well populated server. It must be very difficult for groups on low-pop realms to be suddenly made short-handed.
I mean, even if they made a 7/10, 18/25 style same realm requirement, it would certainly be nice to be able to fill holes with people from other realms.
What are the drawbacks that I'm not thinking about?