I thought most/all that mythological stuff in the last season was pretty stupid, but I still loved the characters and the show.
They were getting close to going into Episode 1 territory (the science behind the Force) where maybe they were telling a little too much stuff I thought. I don't mind the cork, and I'm fine with not knowing anything else about it other than unplugging it is bad news. The reveal of the golden cave in the Jacob episode I thought was a little lame at first, but watching that episode again I ended up liking it more. I thought they did the best job they could trying to give us the Jacob/MIB mythos (how Jacob ended up being island protector, how The Smokey Monster was born). I accepted pretty much all of it.
I liked how the numbers were basically phone numbers so Jacob could dial into people and see what they were up to in the light house. The temple was a bit strange, but it's whatever.
I'll admit the show does have a lot of weird holes when you look at the entire thing (Ben should have known what Smokey was because it seemed like everyone else did), but I loved the hell out of the ride.
William, I don't know how much of the ending of the show they really had planned. I mean, the ending they had planned from the start was probably Jack dying, seeing the plane fly away, and his eye closing. Who knows what their definition of "the ending" was when they say that. Though they obviously had the whole death waiting room planned since they introduced the idea of the constant in Season 4. I'd be willing to say maybe they even had it planned back when Desmond was having flashes in Season 3, but The Constant really did it. So I'm completely fine with the death ending too, because they hinted at it from two seasons ago.