Azuriel of In An Age posted a long series of long posts about the correlation between WoW's difficulty and subscription numbers. For some strange reason they appeared in my feedreader all in one day, although the previous ones are months old. You can find the latest one here, and work your way backwards. They are very worth reading.
As a scientist I find it particularly noteworthy how Azuriel demonstrates how you can't just take your pet peeve about World of Warcraft and declare that this is why World of Warcraft lost 2 million subscribers in 2011. Otherwise I'd like to join the crowd and state that it is obviously the fact that WoW allows addons that made WoW lose 2 million subscribers, while SWTOR without addons can't be far from having gained 2 million subscribers. You see how easy it is to propose causality when there isn't one?
Nevertheless I would say that Syncaine's statement "WoW is bleeding accounts because people are finally realizing that being handed everything with minimal effort and no risk is, in actuality, not that much fucking fun over the long run." as quoted by Azuriel contains a grain of truth, only that I believe most people are looking at the wrong part of WoW to find it. Everybody is always discussing raid difficulty, and as Azuriel points out, if raids being too easy killed WoW, the timing is curious, as raiding was harder at the time WoW lost those subscribers than during the previous expansion.
However I did notice that when Blizzard remade the level 1 to 60 game in Cataclysm, they tuned it from being very easy to being absolutely trivial. In contrast I enjoyed the significantly more difficult leveling game of Star Wars: The Old Republic considerably more. So why I wouldn't say it fully explains the loss of subscribers, I do think that the lack of any risk in the leveling game of World of Warcraft since Cataclysm is at least one factor to it.
If you want to know how the difficulty of the raiding endgame affects subscription numbers, I'd advise you to wait 6 months. Right now, Star Wars: The Old Republic is really successful. If that changes abruptly after people hit the level cap, we can make a more accurate statement about how the accessibility of the endgame affects subscription numbers.