Action combat seems to be fun for most of people who has given Tera and Neverwinter a try.
For those unfamiliar with the system; You don't tag your target portrait and nuke it, you aim, making the fight more realistic and skills related. You have won't be attacking a shield wielder from the front and you won't allow your enemy to get behind you.
WoW combat system is radically different and changing it would probably take as much dev time as an entire expansion I guess.
In my opinion, developers should consider this. Instead of pulling out a new expansion where the core is the same as always, new dungeons, new bg's, advanced lore and lvl cap, etc.. maybe they could work out this new combat system which is quite successful. In fact, Blizzard is already working towards the correct direction with the new character and npc models in order to "update" wow to the current times.
Would you like to see World of Warcraft with a new fight system? combat action like that's it.
Think about it. I see many pros but I can also see many problems since this is a really huge and critical point of the game.