It is difficult and simplistic to breakdown what makes the greatest fighters in Mixed Martial Arts so great into just one skill, but one thing you notice about the elite fighters that makes them stand out from the rest is evasive ability. Fighters like Fedor Emelianenko, Lyoto Machida and Anderson Silva are known for having this skill well developed.
Lets take a look at Fedor Emelianenko. Both he and Machida were top on the list of least hit fighters in the sport and that's not an accident. They spend a lot of time training to avoid strikes. And when they're not getting hit, they can use this time to counterattack their opponents. In his fight against Arlovski, Fedor seemed to be getting hit, but upon closer inspection you can see he was evading the full damage of the strikes with his own blocking/evading maneuvers and finishing Arlovski with a knockout the second he made a mistake.
Machida not only believes in elusiveness as a practical skill but philosophically it is the spirit of Karate. Unlike Muay Thai where the name of the game is damage via the most forms possible, defense in karate is just as important as the striking. In interviews Machida emphasizes the importance of distance in Karate, which gives him a defensive and offensive advantage over his opponents.
The skill of evasion not only helps the elite fighters stay conscious but imagine the mental challenge this poses to their opponents. A fighter that you cannot touch is invincible in your opponents eyes not to mention the fans.
While I do not believe mastering elusiveness will automatically propel you to elite MMA fighter status, it is in my opinion a key aspect of Mixed Martial Arts that will always separate the elite from the rest.
Fightmetric's Elusiveness Statistics:
Machida demonstrating Karate evasive maneuvers:
Anderson Silva evading Forrest Griffin: