Besides the review misconduct, I do think a lot of people's issues with Amazon stem from purchasing no-name products from unknown sellers, an action that is made horrendously easy by Amazon's UI. I've purchased tens of thousands of dollars worth of items from Amazon, and I have had a perfect experience by following two rules: (1) always buy from Amazon (not a marketplace seller), and (2) always buy a reputable brand.
Others have mentioned the issue of co-mingling inventory, and while I believe it's an issue, I've never come across a purchase from Amazon that looked anything less than legitimate.