Slightly disappointed with that last episode. There were a lot of good moments throughout the season, it just went... too dark, I think. Mary's death seemed cheap, especially with her "appearance" later on in the season. It seemed to be done only to see Sherlock and John's reactions. And Eurus just seemed flat and unrelatable. I was intrigued after the conclusion of The Lying Detective, but her characterization was not done well. Honestly, with all the "tests," she felt like a wannabe GLaDOS with less than half the character. The gravestone key was a huge stretch, as it just didn't make sense. Flashbacks show the gravestone to intrigue Sherlock at a young age, and the whole waiting-out-in-her-room thing was only a recent development. Having that be the grand answer of the episode just felt insincere. Death was very cheap throughout the whole thing, and I think it almost lessened the impact of the Redbeard twist. That twist was still shocking, but distanced me even further from the sister. I liked the beginning scene with Mycroft, though. The distinct lack of a cliff hanger had me absolutely panicked, wondering where on earth it was.
This whole thing is out of order, but The Lying Detective was a fairly good episode. Ever since reading it, I have always wanted the original short story The Dying Detective to be recreated in Sherlock, and this was it. It didn't particularly stick to the original, where there is no millionaire serial killer, but I could totally see the borrowed elements.
Synopsis (with s4e2 correlations in parenthesis): Mrs. Hudson, worrying about Holmes' failing health (drug use), demands that Watson come inspect him (recklessly drives straight to John with Sherlock in the trunk, which was btw one of my fav moments in the episode). Holmes does appear to be dying, and a tad bonkers (aforementioned drug use, and effects thereof).
As that's where the correlations end, I won't spoil the whole thing for you, 'cause you should definitely read it. It is one of my favorites. The episode was good, just a tad dark for my tastes.
I personally loved the Lying Detective and The Final Problem. You're right about the gravestones, that was the most confusing for me. The entire ending was very rushed. But I fell in love with Eurus. I started out hating her, but towards the end I just wanted to give her a huge hug. I was kinda disappointed that Johnlock didn't end up canon. And Mary's death broke my heart, though it did seem kind of thrown in there. Overall though the season had its ups and downs.