The militant Suffragettes. Either you think that they are sympathetic (and Truth in Television) and acceptable given the current state of gender politics, or you think it's sexist and it hurts the feminist cause. It's further complicated by the Fridge Logic that their methods wouldn't have done anything to help anybody but themselves, which makes hard to understand how they're supposed to help women in general, which the episode claims they are doing. At best they provide a mythology copycat killers can use to cover the murders of their own husbands (which is exactly what Sherlock suggests as the reason for further murders early in the episode). This however carries a dose of Fridge Horror, as there is also nothing preventing it from being misused to kill innocent people.
Even once one thinks about it from the perspective of the whole thing being a ridiculous product of Sherlock's drug-addled hallucinations, the question is what that means itself — does this show Sherlock's deeply disturbed ways of thinking about women and complete lack of ability to relate to them in a way that makes sense, or is it just incredibly random and not meant to actually mean anything (all the drugs talking), or a combination or something else entirely?
Broken Base: The episode itself is either a brilliant and clever experiment the likes of which are to be expected from this show, or an utter mess that adds nothing to the series as a whole and indeed puts a damper on the excitement for Series 4.
Hilarious in Hindsight: Close to the beginning of the episode, Holmes thoroughly mocks the idea that any of his cases could involve such a hackneyed trope as a "secret twin". Two days after the special's original broadcast, the first episode of a new series of Endeavour aired, with the solution depending on the revelation that one character had a secret twin.
Older Than They Think: Many commentators compared the episode's layered simulation conceit to Inception, apparently unaware Steven Moffat used the exact same trick in Jekyll, another modern reinterpretation of a Victorian novel that has the main character mentally simulate Victorian times (to have a conversation with Mark Gatiss).
Unintentionally Sympathetic: Sir Eustace Carmichael is clearly intended to be an Asshole Victim, at least in universe, but the worst he is shown to be onscreen is a Jerkass, and even then much of his screentime is spent drowning in obvious guilt and terror.