The biggest one is that KDP doesn't support as many languages as Createspace did, so if you were publishing in multiple languages, you may find you can't publish the language you want. Other issues I've heard of anecdotally is in printing quality, though, frankly, I've seen poor quality printing on occasion from every POD provider--the issue is in how the company addresses concerns, and I haven't heard anything on that front. I understand the algorithm KDP uses to calculate spine width is not as accurate as that used by Createspace (they calculate pages in multiples of 10, rather than using your exact page count), so there's a little more possibility for shifting of the cover when printing, leading to titles not quite being centred or whatever, if you don't take this into account. And, of course, if you want to change the cover of your previously Createspace book, you have to use a different template, because the KDP cover will be a different size.