Here's the scenario: it's Saturday afternoon. I have my level 80 hunter around, and wouldn't mind running a Heroic instance for some dungeon rep or a little badge grinding. But my more pressing need at the moment is to level up my paladin, and so under the current LFG here's what I do: get on my pally, log into LFG on some instance I don't really want to run, like Sunken Temple, just so I can listen in on the LFG chat while I level. Eventually someone jumps into the LFG chat, asks for a DPS for Heroic Halls of Lightning. I send a tell from my pally saying my hunter would be perfect, they say great, I log, and I'm off and running. In other words (in this case, Valdesta's), I'm lurking.
Cross-realm groups, of course, throws a wrench in that whole plan, because even with a citywide channel, there's no way for me to know that someone else on a different server wants my hunter to run a Heroic unless I'm logged in and waiting on my hunter. Blizzard's gamble, of course, is that with a cross-realm system, there will be a much bigger pool of dungeon runners to run instances with, and instead of waiting around on my pally until I find a group, I'll just choose an instance for my hunter, get into one right away, and run whatever I want any time.
But of course, in practice, I doubt that'll happen -- especially if I'm playing at off times or off instances, it's hard to believe there will always be a full group ready to do what I want to do. Which means I'm back to standing around in a city on my hunter, when I'd really rather be leveling my paladin.
So what's the fix? Honestly, I don't know. The ideal would be a cross-realm LFG channel, but that seems either impossible or really tough to implement from Blizzard's side. Maybe there could be some kind of automated channel to join, that would broadcast which groups are forming from other realms, or maybe a reservation system, where even people on alts could "sign up" for a group and then have 30 seconds to log onto a main or lose their spot.
Or maybe it's all much ado over nothing -- maybe Blizzard's new system will work great, and everyone will have the group they want every time they need it. But I'm thinking that a little while after patch 3.3 goes live, we'll see the LFG channel back in action even outside of cities. If history has taught us anything about LFG, it's that people who are looking for a group really, really want to chat about it, even when they're sitting on alts or doing something else.