As a store they're... frankly terrible. Search is complete garbage. Filtering is clunky. The product categories are an absolute disaster. And it's often impossible to tell genuine product from counterfeit (a fact that, by the way, has me terrified for folks inadvertently buying fake solar observation glasses for use during the 2017 solar eclipse) It's honestly a terrible shopping experience in almost every way imaginable.
But through direct supply and their fulfillment program they can get products to consumers insanely fast for zero cost (edit: to the consumer). IMO, that is what puts Amazon above the rest. With Prime I'm honestly dismayed when I see an arrival date that's more than 2-3 days out... that's the level of expectation they've set in the market.
Of course, competitors are starting to match them. Walmart, for example, is really advancing in this space. But IMO Amazon is still the king of logistics.
For now, anyway.