Obviously the main reason for people to transfer servers is WvW success.
I have a guild of friends about 10 people and we are on Blackgate. I would happily transfer away to a lower pop server for free, since getting into WvW or main map for PvE is basically impossible anyway.
Biggest problem: Guild influence does NOT transfer. It took us a LONG time to unlock the biggest guildbank and bounties. For me this is the biggest deterrent from transferring.
If anet allowed free transfers to lower pop server AND free transfer of guild influence, guild bank and its contents (only to lower servers) for me that would be a massive incentive to switch. I don’t know if this is true for others.