That's why I've always said it's difficult to gauge how "great" a wrestler is or how over said wrestler is until they get to the WWE. Alot of indy wrestlers can only be great if they can wrestle a certain style, and usually said style is a dangerous style and not conducive to a long career nor to that of their opponents. Without an extremely "strong style" match, it's hard to say Nakamura will be anything close to what he was in Japan.
And that's why I've always believed (over the past 14 years) that Styles is the best wrestler in the world once he walks between the ropes but even then, I had a qualifier that it's hard to say he's the best until he gets into the WWE ring and can get it done. Once AJ stepped into a WWE, the transition was smooth as butter and he didn't even have to say a single word. He was the best in ring wrestler in TNA, he was the best in ring wrestler on the indys, he was the best in ring wrestler for two years in Japan, and now is proving it in the WWE. No matter where he goes, no matter the rules or ring/wrestling etiquette, AJ has proven he gets it done any and everywhere. On a side note, that's why it's hard for me to say Omega is better than AJ and Omega doesn't appear to be stupid so he will likely never come to the WWE.
Now back to Nakamura, while promo skills definitely help, all you have to do is watch him in the ring, no volume, and you can tell the guy is gold. Nakamura is very much a hit or miss in the ring and add that to little to no mic skills, it's going to be extremely hard to get over. And when you work with AJ and your match isn't even as good, nor entertaining, as Shane's match against AJ, the issue may be you.
Lastly, it's like what Jericho said. There is a WWE style that you will have to learn and hopefully Nakamura can learn it and adapt to it but I'm not confident it will happen. His style, his matches, his body just doesn't blow you away and you don't come away from his matches in a bit of awe. Now I'm not saying he's Warrior because he's 10x the wrestler Warrior is but the best part of a Nakamura match since he's been in the WWE has been the entrance...and I even enjoyed Nakamura when he was in Japan