Wellingtongoose on livejournal has done some pieces of meta about Mary Morstan and what might or might not have been her reasoning behind her actions in His Last Vow.
For those of you who have seen the ep and have found themselves really not fond of Mary because of what was revealed in that ep, this meta might offer you a different viewpoint that makes it easier to reconcile yourself with those revelations.
If you haven't seen the ep and don't want to be spoilered, please avoid this meta as there are HUGE HONKING SPOILERS for His Last Vow contained therein.
There's also some meta about Sherlock and what could have been going on physiologically in the episode.
What Mary did - and why - part 1
How Sherlock did what he did
What Mary did - and why - part 2