Here's what did not work:
1. The Bloody Guardsman solution: He was stabbed and marching around ... and didn't even know? I don't care how tight his belt is, a person would know if he were fatally stabbed in the gut. He definitely wouldn't take off the belt and start shampooing his hair in the shower without noticing he's bleeding everywhere (even the flash near the end of the episode acknowledged this, with the guardsman immediately looking alarmed and weak after taking off his belt). And why was the guardsman considered a successful test of this kill-method anyway? The guardsman didn't die, he was sent to the hospital. I'm not saying an original and brilliant locked-room mystery is easy to create, but with all the exceeding dialogue cleverness on the display it would be nice if the mystery solution was more on par with the rest of the story -- they've had two years to come up with three of them and the entire Doyle library to tap as inspiration!
2. The potential victim. John's ex (not previous!) commanding officer didn't have much of a part to play, mainly looking stiff and stern. It helps a mystery if we care a bit about the victim and he wasn't particularly interesting despite other characters talking him up. Our biggest concern about a murder at Watson's wedding was that the memory of his wedding would be spoiled.
3. Watson's freak-out. Being denied access to a stranger's corpse in the shower caused the normally cool and slow-burning Watson to practically start screaming, "Let me examine this body!" Just didn't feel earned (though as it turned out his urgency paid off since the person who found him was wrong about him being dead).
4. The silly walk. Sherlock marching into Buckingham Palace wearing a guardsman's hat while the theme song played. First: Where did he get the hat? Did he kill a guardsman himself? One of the biggest eye-rolls in the show's history.
And finally here's something I'm unsure about: Making Holmes and Watson so deeply and openly affectionate at this point. There has been so much effusive heartfelt mutual praise and warm, fuzzy emoting between the leads during the past couple episodes, it risks draining the show's central snappy tension. Holmes has almost entirely shredded his "high-functioning sociopath" mask, while Watson is full-on love-bombing his crime-solving soul-mate. When Mrs. Hudson talks about Watson finding the one you really click with, it was momentarily unclear who she's referring to. If Mary and Sherlock were hanging off a cliff, it feels like Watson would save Sherlock (if only to prevent him from faking his own death again).
So the big question is: Where does Sherlock go from here? Can the show go back to solving weekly mysteries after subverting them so well? Can it switch back to a drama after becoming a comedy? And most of all: How can Sherlock and Watson's relationship continue to grow when it's radically gone from prickly friendship in season one to a stronger friendship in season two to a bitter break-up last week and now to a love fest?
I realize I'm setting myself up here for reader reactions like, "What's wrong with it being funny? What's wrong with Sherlock and John loving each other? What's wrong with having a unique episode?" Nothing! There's nothing wrong with the episode. It's a great episode. It will probably be many fans' favorite episode ever. But like Sherlock spoiling a happy occasion by thinking too much about it, I'm says these shifts are interesting to note and come with a potential price tag. Weddings are normally the climax of movie for a reason; they feel like an ending. This episode is the first Sherlock that also felt like a potential series ending. Some fans might not want Sherlock to ever end, but if this was the very last episode, I suspect many would be satisfied. Sherlock and Watson met as broken men. They have both been made largely whole. They solved cases together, and have now deconstructed that process. You can add new spats into their relationship, and you can have them solve more cases (what was in that matchbox?), I'm just unsure if this show can operate at the same level following this natural break-point. This is, at the very least, a challenge for the writers. They are self-aware enough to know this -- characters repeatedly referenced that marriage changes things. Even Mycroft (played by writer-producer Mark Gatiss) points this out to Sherlock. And Sherlock says, "I prefer to think of this as the beginning of a new chapter."
Let's see how Sherlock tackles that new chapter. We have one more episode before we take our next break. And the game is ... something.