My reason for leaving Prime was that I was buying more stuff from 3rd parties who had free shipping for all than from Amazon or Prime 3rd-party sellers.
Also it started costing me a lot to just click the default buy option and I was having to do CSI-level due diligence on every order if I didn't want to end up with a counterfeit and/or pure scam. Several items I ordered were just straight up fraud that required annoying customer service interactions.
Losing the 5% credit card discount seemed painful until I realized that I buy 30-50% less stuff I don't really need now. Or that buying elsewhere I save much more than 5%. I'm surprised to be saying it, but I've returned to eBay for a lot of purchases and been pleasantly surprised.
I did accept the 30 days they just gave me, so I have Prime again for the moment, but will not let it renew/charge. I'm doing some household crud so having one day shipping on cheap crud for a month is probably going to be somewhat helpful as long as I didn't pay for it to begin with.
- Prime Exclusive Items was the big one. Amazon arbitrarily selects everyday items that people regularly order like diapers and razors and Blu-ray movies and marks them "Prime Exclusive", meaning you can't order them without a Prime subscription at any price from Amazon. This isn't just Prime exclusive pricing, and it's usually not even being sold cheaper than other retailers. It's literally Amazon just saying "f--- you for not having Prime, shop elsewhere!" And often, I've done just that. This has been going on for years, Amazon has not so much as commented on the practice despite massive forum threads about how arbitrary and punitive it is.
- Annoyingly complicated return mechanics. Around Christmas I bought a thing, and then bought a better version of it, both on Amazon. This was a Blu-ray set, so a small item, and it was shipped and sold by Amazon. I was confident I could return it because I'd seen all the ads about free returns at Kohl's. But it turns out, only select items can be returned at Kohl's, even if they're, you know, small items sold by Amazon like a Blu-ray. I had to pitch a fit to get a return label to drop it off at a UPS store, because the Amazon guy couldn't do a return at the Amazon return desk at Kohl's, when the only reason I was returning it was because I bought an even more expensive version on Amazon. Amazon has half a dozen return options, and it's a toss up which one will be available for the given product ID you want to return and how much it will cost to do so. If I buy from Walmart, I can just take it to a Walmart store and it's done.