The PvE To PvP character transfer was something a lot of players were asking for since a long time ago but it was something Blizzard just never gave. I actually thought they never will since the service could very well be abused. But alas, it is officially here! PvE to PvP Paid Character Transfers are now available. the FAQ on this matter can be found here.
The reservation behind not allowing this in the past was that PvP servers are different from PvE servers. Players who play in PvP servers will tell you that it can be a bit tough leveling up since there's always that risk of getting ganked by members of the opposing faction. Because of this, certain zones like STV, Hillsbrad Foothills, and others were very risky to quest in at peak hours. I play in a PvE server and I know that it's pretty comforting to know that the Alliance guy standing in front of me waving at my horde alts won't just attack me. I also know that it sucks for your low level alts to meet an enemy level 70 in contested territory.
Because of this, I've always held back on playing on a PvP server, Getting ganked just isn't my style and I prefer to keep my PvP activities in the Battlegrounds and Arena where they belong. With this character transfer service, what's keeping people from leveling up in a PvE server hassle free and then moving to a PvP server when he feels he's better prepared to take on the ganking? Maybe this issue isn't as important as it once was but I feel like this is one thing Blizzard should have held on to. Of course now that the option is available, I'm very sure a lot of players formerly stuck in PvE realms waiting to get out will want to avail of this service.