I disagree. He has an amazing eye for action, and is a fantastic visual storyteller. I wouldn't say he's a top ten director, but to say he's not great is a bit much, IMO (although that largely depends on how one defines "great").
He gets a lot of shit for aping the styles of other directors. Meanwhile, people can't nuzzle Tarantino's balls enough when literally everything he does is a ripoff of 70s Hong Kong cinema, blaxploitation, and spaghetti western. The only difference is that he makes "art," so his aping gets to be called "homage," while Abrams makes general audience fare, so it's pap from a copycat hack.
I think the difference is that the closest thing Abrams has in the form of a signature style is a joke about lense flares whereas Tarantino, despite primarily making pastiches of various genre films, still manages to make his movies feel like they belong solely to him. The biggest running criticism about TFA is that it comes across as JJ Abrams Presents: A New Hope. That doesn't make it a bad movie in the slightest, but you can't say something similar about any of Tarantino's work. It's the difference between being inspired by an earlier work and being a slave to your inner fan. Abrams' greatest strength comes from his visual panache but that alone doesn't make him well rounded. But then again, I'd say he's more well rounded than the majority of directors in Hollywood.