Not specifically registration, but some sites just make using them so annoying that, a lot of the time, I just give up in frustration and take my business elsewhere.
* Any site [like eBay] which uses reCAPTCHA. But even more so, any site which uses reCAPTCHA as a 2nd stage in the login process, AFTER I've already correctly entered my username and password.
* Any site [like PayPal] which insists I use 2FA, but only offers SMS as the two-factor option. I live in an area with no phone signal. So, every time I buy something using PayPal, I literally have to go outside and climb part way up a hill opposite, to receive the damned confirmation code.
* Any site [like eBay & Amazon] which greets me by name, shows me my purchase history, lets me view my account, add things to my shopping basket, etc. But then suddenly asks me to login, when I try to do some trivial thing, like change a preference.
