The way overflow worked and works now is that once the server version of a map is full, people move into overflow. Overflow version of maps are not server specific and can contain people from any server whose default map was full or who used "join on member" in their party window to move into overflow.
This is how TTS would do multiple tequatl kills by spawning overflows on a particular server then just inviting everyone who wanted to go into the overflows so people who couldn't guest didn't have to.
Now the megaserver sounds like instead of being a random mishmash of people and invitees in a overflow map....they try to group you via your language, guild, etc. So it will make the base maps on smaller servers actually have people in them at all hours instead of just primetime for that server.
And I think people who've grown used to the other server system are going to be upset for awhile. And all the "rivalries" between wvw competitors are going to start showing up in map chat everywhere. People already spend hours trolling via secondary accounts and going on the forums and spewing crap all day. So here's hoping Anet groups via color groupings like they do in Edge of the Mists so you won't have to see as much of it.