It is a very different game now from Vanilla.
But it's still easily one of my favourite games of all time (if not my favourite). And the thing about the level of difficulty is that it's gated to people's tastes and requirements.
If you want the normal/heroic raids you still have that, and they are still a challenge, but then if you are more a casual player (like me) then there is the LFR option.
Anyway, i'll continue to play it as long as it's enjoyable. 5.4 has been great so far (loving the Timeless Isles) and I can't wait to get my hands on Hellscream.