Ok this is incomplete, I know about transfer in Europe but not in China (should be the same way though);
*you can only transfer from the servers selected by Blizzard, usually 3 among the over overcrowded realms towards 3 empty ones.
*Transfer can only be done during transfer period, which happens twice the first year. These events can be seen as totally exceptional.
Of course wow is only a game, but lets consider transfer from Blizzard point of view: transfering a lvl 60 with maybe a lot of money will "break" the economy of the new server, he might bring items of unvisited dungeons and more than the rest, he has a pvp rank that completely mismatches the new honor ladder. Its too complicated for Blizzard to handle all these questions, its obviously the best way not to provide transfer.
I admit as a non hardcore gamer, its quite not convenient but here we are. Now things may be slighlty different here, dunno more about this...
田畕畾畕田 - Mage Herborist/Miner/Grinder