Great new mechanic to work with Blizz coins.
1. The new Blizzard store is ALSO there for Blizzard's new F2P games Hearthstone and the upcoming Blizzard All stars. Because these games are F2P and need a revenue model.
2. By making the payments through Blizz coins you actually create a bond between all Blizzard (WOW) games.
3. Hearthstone developpers stated already you can earn these coins by winning matches in HS.
So it is quite clear that the cash shop is not a transition to a F2P WOW, but it is promoting the play of Blizzard games by creating a common and virtual coins system.
With the coins you can buy new HS packs, special heroes, new stuff and of course new fluff WOW mounts too.
So while earning coins through play, Blizzard avoids the trap of F2P mechanics in sub based WOW, but the WOW games besides it contribute to the coins/payment system.
So how did they call Hearthstone again ? "The game that will change the World (of Warcraft).
So this has been planned a long long time ago.