Their site's UX is amazing. The aesthetic design could use some work, but the actual interaction and interface design on amazon is pretty great.
Their goal isn't to look pretty, it's to sell stuff. There have been many times when i've been looking at something on amazon, clicked the one-click purchase button, and then hours later realized i wasn't really sure i wanted to buy it, Amazon just makes it far too easy to give them my money.
Compare their site to Newegg for purchasing computer hardware. Newegg's filtering system blows Amazon's away, making it possible to easily find what you are looking for.
Amazon likes to give me similar hardware when I search for a part, cluttering up the results with things I don't want. If I search for a 7850 GPU, I don't want results for a 6950, 7950, 6870, 7770, 7870, Intel 3570K CPU, GTX 670, Intel 2500K CPU, GTX 480, 6850, GTX 560 Ti, and GTX 560. That was all just on the first page of search results.
Granted, you do want similar products in other cases, but the same kind of failure applies elsewhere.
Another annoying thing is the duplicate items as separate listings. Sometimes I will go and look for something like a Water Filter replacement and find 3 or 4 different listings for the product with multiple sellers under each.
Using Amazon to find a specific product is an unpleasant experience. Thankfully once you find the product you are looking for everything else is great.
Their goal isn't to look pretty, it's to sell stuff. There have been many times when i've been looking at something on amazon, clicked the one-click purchase button, and then hours later realized i wasn't really sure i wanted to buy it, Amazon just makes it far too easy to give them my money.