Of all the components required to build a successful WWE superstar, a strong character might be the most important. A performer can be the best athlete in the world, but they'll find lasting main event success hard to come by without the right gimmick, even in the modern era.
The company have largely moved away from the outlandish gimmicks of old, but this is still 'Sports Entertainment,' not traditional wrestling. Personality is key, but it's no longer enough to saddle a wrestler with a paper-thin concept and send them out on television. Nobody demands Sopranos-esque levels of character depth, but today's audiences are smart, and won't stand for the one-note gimmicks of old.
WWE are wise to this, and while they don't always succeed, they've at least attempted to turn the bulk of their roster into multifaceted, engaging characters. There are plenty of duds (Jinder Mahal), and a handful of great gimmicks gone wrong (Bray Wyatt), but that doesn't detract from those at the other end of the quality spectrum.
WWE's inconsistent creative team miss more than they hit, but they're absolutely nailing these characters at the moment, and they stand as the best in the company.