Some of them:

* Payments cannot be reversed, even when coins are sent to addresses that non one has the key, or as a result of crime.  That makes it unacceptable for most comercial and individual uses.

* Users are anonymous, which makes it much more attractive to criminals (much more so than cash, for obvious reasons).

* The fixed supply created the expectation of fabulous gains that turned it into a speculative pyramid schema

* That "ponzification" in turn resulted in extremely volatile price, that makes a bad currency.

* The block reward and the market price were too high, leading to a hypertrophied and unsustainable mining industry.

* For that reason, the cost per transaction is way too high.

* Mining got inevitably centralized (last time I checked, the top 5 Chinese miners had 60% of the hashpower).

* There is no mechanism to reward the relay nodes, which carry increasing load.

* Its design cannot support 1 billion users, maybe not even 10 million, even indirectly.

And a few more.  Before you say it, yes, I know that many bitcoiners see those as advantages rather than flaws.