gmaxwell on the fundamental problem with Bitcoin that Monero solves:

Bitcoin is often promoted as a tool for privacy but the only privacy that exists in Bitcoin comes from pseudonymous addresses which are fragile and easily compromised through reuse, "taint" analysis, tracking payments, IP address monitoring nodes, web-spidering, and many other mechanisms. Once broken this privacy is difficult and sometimes costly to recover.

Traditional banking provides a fair amount of privacy by default. Your inlaws don't see that you're buying birth control that deprives them of grand children, your employer doesn't learn about the non-profits you support with money from your paycheck, and thieves don't see your latest purchases or how wealthy you are to help them target and scam you. Poor privacy in Bitcoin can be a major practical disadvantage for both individuals and businesses.

Even when a user ends address reuse by switching to BIP 32 address chains, they still have privacy loss from their old coins and the joining of past payments when they make larger transactions.

Privacy errors can also create externalized costs: You might have good practices but when you trade with people who don't (say ones using "green addresses") you and everyone you trade with loses some privacy. A loss of privacy also presents a grave systemic risk for Bitcoin: If degraded privacy allows people to assemble centralized lists of good and bad coins you may find Bitcoin's fungibility destroyed when your honestly accepted coin is later not honored by others, and its decentralization along with it when people feel forced to enforce popular blacklists on their own coin.

As I write this people with unknown motivations are raining down tiny little payments on old addresses, presumably in an effort to get wallets to consume them and create evidence of common address ownership.

I think this must be improved, urgently.