Well it pretty much has already been said, but I personally find it straightforward. Kenpachi hasn't learned anything nor did he want to learn anything, simplistically said he just swings his sword around. The fact is that he is just extremely powerful, so powerful that he can even beat captain class characters with just his raw power. But it's also a fact that someone who trains, will become better.
All the other captains became so strong due to their extensive training. So Kenpachi is already in their league just with his raw power. Imagine what you get if he actually learns to harness that power and use it effectively...a monster beyond a monster, which is why Shunsui said that Kenpachi is so important. They weren't able to stop the Quincies with their battle power and that was when the still had Jama-ji, so logically speaking they would be screwed right now. The only person among the Gotei 13 who actually has enough potential to significantly grow in a short timespan is Kenpachi Zaraki. Even if all the other captains plan to train ferociously, it's not like their growth would be enough to make a real difference. After all Kenpachi is the only captain ever in the Gotei 13 who became a captain without achieving bankai and he still can go toe to toe with most of the other captains.
That can mean, but not necessarily, that Kenpachi might achieve bankai, but that would just be a part of it.