I thought that Sally was one of the most interesting characters in Sherlock Series One and she was one of my personal favourites. I love the fact that she is a strong black woman, working a professional job, living an independent life, and working her way up the career ladder at the Metropolitan Police Service.
There was also the very interesting Sherlock-Sally dynamic. I was firmly in the camp that there was a backstory between Sherlock and Sally.
Their first interaction when Sherlock arrives at the crime scene was very telling.
I thought that either:
a) Sally had a crush on Sherlock (what woman wouldn’t?) and he was indifferent or more likely rejected her in his haughty manner, or
b) they had a brief relationship where Sally invested more than Sherlock and it ended with Sherlock ending the relationship.
Otherwise, how could someone as intelligent as Sally resent/hate Sherlock so much unless there had been something between them in the past? Sally SO acts like a spurned lover - hell hath no fury like a woman scorned and this is most definitely the case with Sergeant Sally Donovan!
What reinforced this idea for me was Sherlock describing Sally as an “old friend” in a very sarcastic manner and his pointed – and very ungentlemanly - remarks to her about spending the night at Andersons.