"That segues in nicely to this question: Cross-server gameplay. It's convenient, but do you think that it runs the risk of destroying server communities?"
As for the community question, I used to ... I think that 5 years ago, I would have answered this question differently than I would today. I was all about preserving the small realm communities, but already... Well, look at Battlegrounds, it's a good case in point, because it doesn't diminish social relationships that matter on a realm. Sure, everyone can bring up "that one guy" that they know, the ninja looter who stole his stuff. But I think your real community isn't the whole realm, but it's your guild and the friends you group with, and the cross-server LFG won't undermine that at all.
I'll start with a small bit of opinion: I don't mourn the death of small towns IRL. I am annoyed by politicians' pandering to 'small town America' as if it is some unrepresented demographic made up of honest, hard-working, moral people while everyone else is a big-city banker or homosexual artist, both of whom hate God and freedom. Why do I bring this up? Because then you can play "find the contradiction," since odds are at some point I'll get sad and nostalgic for the 'small towns' of WoW. Oh. I already did that!
"But I think your real community isn't the whole realm, but it's your guild and the friends you group with, and the cross-server LFG won't undermine that at all."
This is where I see risk: redefining community. As we join larger communities we tend sometimes define our community in reverse: making it smaller. While we always had our guilds and our friends and those were special, they were not the entire definition of community. We did have servers and guilds were influential on those servers.
As the community expands outward to include the entire battlegroup, our perception of community cannot keep up. We cannot know the entire battlegroup, we cannot even recognize the guilds. Our server is diluted. In reaction we invert our community and restrict it to friends and guilds.
The result is that if we don't have a guild, we're lost. We must find a new one or risk being outside the community. This risks isolating people. Isolated people are dangerous because they have no social institutions to follow and will tend to act out of short-term self-interest, with harmful effects on those around them. In real life this is theft and all manner of crimes while in WoW it is ninjaing and flame wars.
Sorry, this is getting too slippery slope. We're not on the edge of chaos. This mean that small towns keep us moral while cities encourage lawlessness. It does mean that we need to keep an eye on social institutions and be aware of the possible trouble from transitions between them.
The cities of WoW, battlegroups, can have the same benefits as in real life. These benefits overcome any losses. Sure the social structure gets rearranged, but I believe that this actually encourages involvement in stabilizing structures. In real life this takes the form of what would otherwise be unsustainable minority populations being able to concentrate enough to form a self-sufficient community and from these build themselves up and approach the outside world as equals. For a good example, look at Jewish enclaves in the US where they built themselves up into a powerful group whereas when they were out in the European countryside people tended to kill them as the most convenient cure for a poor harvest.
Cities allow small activities to gain the population they need. Can you imagine a grand art museum in a town of 10,000? It's possible when that's 5 million instead. Cities allow residents to poll common interests to do what they could not otherwise and in that process they create a more varied and valuable culture than the pockets of homogeneity of small towns. In WoW terms this means a handful of scattered lowbies can run an instance together. This means the smaller faction can pool together across servers to fill a BG. This means getting groups for something other than the latest raid and daily heroic.
Overall I'm glad to see the benefits of the larger community, but we must be aware of the costs of this community in order to minimize the downsides and get the most possible benefit. I'm not going to stop missing the faction feel of old AV and knowing the names of people on my server for something other than trolling trade chat, but I can accept that this are smaller losses than the gains of more frequent instance groups. And perhaps we'll even see the full benefits of diverse populations and options as more people find that the perfect guild was waiting on the other side of LFG all this time.