At one point I tried rolling on a low population server. I had a good group of friends to play with, but it was hard to PuG anything. I wound up transferring two characters from that server to my main realm. Several friends did the same. Unless it's an entirely new type of server, I think I'll stay where I am.
Some realms like Ghostlands have really taken off, others remain on the recommended and low population lists indefinitely. Hats off to those willing to colonize new and underpopulated realms, I have no idea how you do it.
What would make you change servers?