The most painful thing I've encountered is having to register on multiple different e-commerce stores and fill in all my personal info again. Most consumers don't want an account, and most merchants allow you to buy without registering an account, but some don't do this and you have to jump through hoops just to buy something (filling address, e-mail, phone number, and confirming e-mail, etc). It adds too much friction, which is why people prefer Amazon. All their details are pre-filled and third-party merchants just grab those details for orders.