Try this for example. Post some negative thing you have to say about bitcoin, and then replace bitcoin with USD. If the statement is still true, your argument is invalid.
99% of value based on speculation, that speculation driven by marketing using contrived figures. ("If 100% of the wealth was represented by Buttcoins, each Buttcoin would be worth millions of dollars." Yeah... and if 100% of the wealth was represented by my penis, my penis would be worth trillions. That doesn't mean my penis is a currency that's worth the entire planet, Einstein.)
Unregulated market susceptible to pump and dump.
Currency verification can be counterfeited with a botnet.
Savings are not FDIC insured.
Funds can be stolen by remote, untraceably and with no recourse.
Creator owns a large percentage of the currency in existence.
Not usable to buy much of anything and you have to jump through hoops to convert it to a usable currency. (Since you have to convert it to USD to buy anything it's just a skin for USD with a gambling addon)
No physical presence. (Power's out? Can't access any currency. Internet's out? Can't trade currency for anything. EMP or hardware malfunction? All currency gone.)
The computational inflationary model Buttcoins used meant that early miners could leech money from idiots who put real money into the system, with it being hidden by the Ponzi-scheme fictitious return values. Now mining isn't worth the electricity cost and you have a currency whose only basis is the shared delusion that it is worth X amount in USD. It's a currency whose only use is to be traded for another currency -- it can't purchase anything on its own.
Next you're going to be telling me that gold trading at $1700/oz means it's the bestest currency.
Idiots.