I know I'm new, but I just thought that I'd offer my two cents. (:
Population-wise, I personally prefer some kind of middle ground. I like there to be bits of everything there; people who rp, people who don't, serious players, casual players and players of all nationalities. Like everyone's already said, I'd like to be able to play with people who don't rp as well. I don't think there's really a way to prevent over/under-population though, especially when you're picking a server as soon as the game opens. I guess you can go around other forums doing a sweep of what servers people plan to use for a general idea? I just say don't pick the one at the top of the list. >__> /former Alexander player.
On the subject of griefing, I think it's inevitable at some stage. It doesn't really matter what server or whether or not there's PvP, there's going to be people out there who think rpers are stupid (or, apparently, creepers trying to be something they're not, bleh). But without the PvP aspect there'll be a little less animosity, I guess. You have to put up with a bit of griefing wherever you rp, though. One of the main forums I used to use for advertising rps always got attacked by the General Discussion of that forum just for enjoying rp, and we had otherwise no contact with them. Some folks just don't like it.
As for international rpers, there are definitely people who speak other languages who rp. I don't know about MMOs specifically but for some reason there's a pretty large Swedish play-by-post rp community. I'm trying to find out if there are a lot of Japanese rpers atm, but I have NO idea how 'role play' translates in Japanese. .__.;
