As a consumer, Amazon is amazing in terms of breadth, price and ease of use. I'm not claiming that they have every product, nor are their prices always the best.
But their selection is very good and their prices are typically very competitive. And if I'm just buying run-of-the-mill stuff that doesn't require too much research (e.g. a USB memory stick), it takes me about 60-120 seconds to search the site and check out and I'll almost always receive the product on time, 2 days later. Very few other retailers can match the shipping time. Plus, when I have had issues with products, I've generally been very happy with their customer service.
However, from a consumer's perspective, I have two major issues with them:
1) Knock off and counterfeit products - there seems to be more and more of them these days, especially with third party sellers or even fulfilled-by-Amazon items, since it seems like they might co-mingle inventory. If I'm making a large purchase where there is risk of buying a counterfeit item (e.g. Bose headphones), I tend to go elsewhere these days, unless Amazon has a very competitive price. In particular, for electronics, Best Buy will match Amazon prices and I trust their supply chain more. I also find that B&H has excellent customer service, a great selection, great interface and are reliable to work with.
2) Suspicious Reviews - No doubt there are a lot of fake reviews on Amazon. When you're talking about the largest online retailer where reviews make or break a product, obviously there are going to be fake reviews. Then there are products with what I'd call "hired reviews". I believe that this is no longer permitted, but this is where a company gives free (or very cheap) samples in exchange for an "honest" review of the product. Naturally, people who are given free (or cheap) samples in exchange for writing a review are going to give biased reviews. Finally, for many items, it could be as simple as friends and family writing reviews of your product, which are also going to be biased. Because there's so much incentive to game reviews, it's gotten to a point where I no longer trust them anymore [1].
As a consumer, what I'd really like to see is a competitor to Amazon with a more curated selection. For example, I don't need 250 different options for blenders. What I would prefer is a site that offers 5 options at various price points which have all been vetted by the store owner. User reviews are helpful, but most users aren't really familiar with the alternatives. I can say that I like my blender, but at most, I have experience with one or two other blenders. However, when a store-owner says they prefer a blender, I have more trust that they have experience with the other models (even if they haven't used them, they've seen the returns and/or customer complaints) [2].
From third party sellers' points of view, I've heard many horror stories of issues they have had selling with Amazon, but I won't get into that right now.
[1] There are two other big issues with user reviews in general. The first is that when a product (or establishment) gets enough good reviews, people buy the product and are inevitably disappointed. This is an even bigger issue with reviews for things like restaurants. For example, say a local diner gets 100 five-star reviews on Yelp. Now, people come across the place on Yelp and they see it has great reviews and so they decide to check it out. But they came in with very high expectations so they are inevitably disappointed and so when they write their own review, it will be lower than it might have been if they had gone in with average expectations and were pleasantly surprised. The other issue with user reviews is that every user has different needs and expectations and many of them won't match mine. For example, say I'm a professional photographer reviewing the Sony RX100 V camera, which retails for about $950 and is popular with amateurs as well as professionals looking for a small sized travel camera. If I'm a professional in need of a very small camera for travel, I've got completely different expectations than an amateur who is using this as their primary camera. For example, $950 might be a lot to spend on a camera and an amateur might justifiably feel that the $500 alternative does the job 90% as well. But for a professional, $950 is almost a budget camera and if it performs even a little bit better, the difference in price might be negligible. Needless to say, the reviews from both professional and amateur users of this camera get grouped together on Amazon, which ends up meaning that the reviews are less helpful for both groups.
[2] This is why I love sites like The Wirecutter, where they examine five to ten options and pick the one they like best. Consumers generally don't do this and so I find a single review from the Wirecutter to be more useful than 1000 reviews on Amazon.