+1 to this. It's a great idea. It's so good that EQ implemented back in '99. I believe they even had cross server channels, as well. I remember listening in (to another server) as Kerafin was taken down.

But back to global channels, if you had to opt-in first, that would be great. A LFP channel, a bazaar channel, a crafting channel (or one channel per class), etc.