I actually believe ESO will become heavy competition for WoW. It's the first MMO game in a decade from a AAA quality developer that is -not- trying to be a WoW clone. It will have a much larger potential market than WoW because it'll be the first next-gen console MMO and a worthy sequel of Skyrim, the most sold console game. The Elder Scrolls fanbase is very very big and much more 'PC active' than even the Star Wars fanbase was. Along with all the positive coverage the game is getting on popular media outlets over the past few weeks (while games like Wildstar seem almost ignored), ESO might very well become the reason why WoW might consider going F2P instead.
Look at the character customisation in WoD, there is none. Then take a quick look at the endless character customisation options in ESO.
Look at how 25m raids cause performance issues due to the old engine in MoP. Then take a look at the smooth 300v300v300 territorial battles in ESO that run smoothly and with better graphics on low to midrange computers.
Look at how they're offering L90s in MoP for a payment to get a questing and leveling experience that stretches out for at most 15 hours of play. Compare this with the 200-300 hours of leveling content in ESO, with everything voice-acted, cutscenes, dynamic and branching storylines. Oh and then let's also laugh with the brawler's guild which does nothing in WoW, and compare it with the Fighter's, Mage, Undaunted and soon to be added Thieves Guild and Black Brotherhood which add roleplay and character customisation.
The 7 or so dungeons in MoP compared with the 50+ public (solo) dungeons and 15+ group dungeons in ESO.
How crafting is a boring grind in MoP compared with the exploration and research based crafting system in ESO which according to calculations would take 2000 days for someone to learn every last useful type of trait and improvement.
The measily Garrison system promised in WoD, compared to the expansive sandbox systems in pvp, economy and justice (karma) system that will be in ESO...
ESO is going to be widely popular in a few months once people start realizing how WoW is a tiny and old game compared to what Bethesda and Zenimax have been cooking up, and honestly at this point in time... WoW is more of a F2P and itemshop game than ESO is too. It makes quite a bit of difference in how the quality of a game is perceived by it's playerbase.
Blizzard have been sitting on their ass for way too long and have not put enough of the money back into the game to keep it up-to-date. Not properly developping a successor next-gen MMO is going to hurt them now too. Well it's too late now... they fucked up and a behemot of competition is right around the corner waiting to kick their lazy ass.