Well, having played extensively in Nostalrius, I can tell you that the idea of a "MEGAREALM" is extremely bad. In Nostalrius, we have had around 13000 people concurrently online on the main server
...and around 16000 people online during China Prime Time.
Here's why a MEGASERVER is a very stupid idea:
1. The server was
UNPLAYABLE during China Prime Time. The systems simply could not take much more than around 14000 players and still provide a lag free experience. As far as I recall, we had a 4 second delay on actions, spells and abiloties.
2. Quests that relied on mobs that spawned on a timer were IMPOSSIBLE TO DO. Case in point, the Hunter Quest. In each demon spawn point there were always several hunters in waiting, typically from the same faction so you couldn't kill them, and there were also several griefers. The end result was that the quest was not doable for a large art of the playerbase for several
MONTHS on end.
3. Word PvP & Honor Requirements: Because there were
OVER 20000 players participating in the Honor System at the same time on the same server, ranking in PvP required an ABSURDLY HIGH NUMBER OF HONOR POINTS. This meant that there were many desperate people for honor points who camped choke points around the world.
For instance, I was attacked at one point in Nostalrius three times in Ashenvale as a 60 geared Hunter by three Horde 60 PvP'ers while on the road from Astranaar to the border between Ashenvale and Felwood.
The whole world was camped. It went beyond the point of "world pvp" to the point of "world pvp clusterf*ck".
I have read so much BS in here by clueless ppl that I really hope that Blizzard won't base some of their decisions on how to run the new Classic server on peope who post in here and in the Retail forums.