Cuba as OJ
This was the only casting decision that was hardest to accept. Everyone else looked enough like their characters and were good enough actors to pull it off. Even Travolta as Robert Shapiro seemed like a stretch at the start but over the course of the show I ended up really liking that choice. But Cuba as OJ was tough because he didn't look any where close to OJ, and wasn't physically big enough. OJ is physically very imposing, and Cuba just didn't have that.
That being said, the one scene where he absolutely nailed it, was trying on the gloves. Just the smug smiling, and little shrugs to the jury like he was saying "I dunno, these gloves just don't fit, it's crazy." was absolutely perfect.