#1. security. When you make a bitcoin payment; that one payment address then forever connects you to your wallet thus allows the receiving person to see all your previous receives/sends, and, all your future receives/sends. Any payment you received, or spent, then crushes your anonymity. You are forever outed; fully exposed. Really, it's no different than everyone in the world watching your bank account; seeing all the payments coming in and going out. Bottom line is you can never send or receive a payment to anyone in the real world. Your bitcoins/activity will forever be linked to you. Chainalysis perfectly lays out the problem.
Security is one thing that Blockchain Technology boast and yet you are saying that Security is one of the thing that Bitcoin should improve. The only mistakes that happen within the Cryptocurrency are the mistakes that are the cause of carelessness. Most money loss within this industry is the lack of understanding on how to approach things and what to do when situation rises like clicking on random website links. Being careless within this industry will really result in a lot of loss and that is why we need to be updated and be mindful of everything that we will see. Dont be scared to ask questions because asking them is one way to understand what we dont know.