Good cinematography, production design, sound, and performance of the lady playing Zinzi. Horrible old-age make-up (check mom of Mandela, or worse still, check the Mandela face when Zinzi visits him in jail... embarrassing and unwatchable). Also, I didn't see Mandela in the lead performance (Idris Alba is not a very good performer, and is not credible in the role).
The story was very rushed, and it leaves one feeling too many details were left out (there was no mention of Oliver Tambo or Albert Luthuli, for example), and some very significant moments merely given passing attention (Sharpeville and Soweto killings, for example). One thing that could have helped is narration, or even flashbacks. One can argue that you can't fit everything in a single movie, but that would be an excuse... I've seen biopics in which feel like they do justice to a legend, recent examples being The Iron Lady and Lincoln.