They've focused a lot on operations and efficiency. They already had 2-day shipping but they've worked to get that down to 1-day shipping where possible. Which is impressive, though 2-day shipping was already pretty good. They've optimized the hell out of their inventory, distribution, etc. They made that Haven joint-venture healthcare thing (together with two other huge companies) to drive down health insurance costs.
Another area they've focused on is new types of business, like cloud computing, buying Whole Foods, getting into video streaming, and hardware devices (Kindle, Fire, Echo).
But the store itself, the web site design and how the store functions, hasn't changed that much. They had a lot of issues with fake reviews, and they eventually did something, but it took a long time. They have some longstanding usability pain points that they haven't done anything about. Reviews are supposed to be about products, not sellers, but people leave comments and ratings about sellers in product reviews because there's no other place to put them (or if there is, it's not easy enough to find). Something (bad UI?) seems to encourage unhelpful "I don't know" type answers in products' "Customer questions & answers" section. If you click "There is a newer model of this item", it often leads to something that isn't. Product categories and the features within them are insufficient or out of date: I can browse the cell phone case category, but under "phone compatibility", Galaxy S10 isn't listed, only Galaxy S9, even though the S10 came out 6 months ago. Similar story with iPhone. And nothing with size variations of phone models (iPhone XS, XS Max, and XR would require different cases).
All of these things would probably benefit everybody if they fixed them. And it's not just a matter of adding polish. There are more innovative things they could do, too. But the attitude seems to be that the store is basically as done as it's going to get.