I think the warning for SPOILERS goes unneeded here, but this list from Digital Spy definitely gives away a few juicy details we wouldn’t know otherwise.
1) The core of the episode is one extraordinarily long monologue: Sherlock’s best man speech is interwoven around extended flashbacks, which fill in chunks of the six-month gap between ‘The Empty Hearse’ and the wedding day.
2) Sherlock has more than a few surprising extracurricular skills up his sleeve. One is napkin origami. Another recalls one of Moffat’s best-loved episodes of Doctor Who.
3) “Either I’ve caught you in a compromising position, or you’re working out again.”
4) Remember Mrs Hudson’s husband who was executed for double murder? We find out quite a bit more about him.
5) John’s stag night is the funniest sequence in the show’s history.
6) “Yeah, but am I a pretty lady?”
7) The whole episode is essentially a love letter to John Watson.
8) A very familiar face from Sherlock’s past makes a brief but rather thrilling return.
9) “Oh, Sherlock. Neither of us were the first, you know.”
10) By the end of the episode, we find out what Sherlock’s “last vow” is.
Wow… speculate away! And here’s the trailer for the episode.