From Slate, some speculation that Caster Semenya sandbagged the 800 meter final in order to avoid further gender-related scrutiny.
After the race, track and field aficionados questioned her tactics. The BBC's David Ornstein said it appeared that Semenya "had more left in the tank." His story quoted BBC commentator Kelly Holmes, who won this event in the 2004 Olympics, suggesting that Semenya hadn't made her best effort: "She looked very strong, she didn't look like she went up a gear, she wasn't grimacing at all. I don't know if her head was in it, when she crossed the line she didn't look affected." Meanwhile, Sports Illustrated senior writer Tim Layden tweeted that Semenya "seemed oddly disengaged most of race and not tired at end."
I watched the race and Semenya's finish was odd...she made her move super-late and was moving at a tremendous pace when she crossed the line. Had she worked her way up to the front before the final turn, she may have beaten the field by several lengths.
Update: Here is a more nuanced analysis of Semenya's effort in the 800 meter final.
Perhaps there is nothing to her performance other than that she runs a more even pace than her rivals.
A comparison between her semi-final and this race is interesting in this regard. In that semi, she went through 400m in just over 58 seconds, 600m in about 1:28 and then closed the final 200m in 29.5s, looking like she had something in reserve.
Tonight, she went through 400m in 57.69s, then through 600m in about 1:27.1, and then closed in a touch over 30 seconds. My point is, her performance in the final was slightly faster at every stage than the semi, until she closed slower over the final 200m. To finish SLOWER than she did in the semi implies that she has little reserve and that she is closer to the limit than she looks. She wasn't actually that fast over the final 200m, it's just that everyone else was very slow!