What makes you say that? As far as I can tell, the only actual downside of it is having to type longer addresses sometimes, but one should really just use the DNS for that. And a bigger address space was needed. Everything else seems better or at least simpler. Autoconfiguration (SLAAC), only one loopback address (which is shorter than any of IPv4's loopback addresses), subnetting, no need for NAT, proper support for multiple addresses per interface…

In practice, most problems with IPv6 probably just come from bad support for it in software. That means they should be improved, not that IPv6 was a failure. Also check that you're not blocking ICMP6 traffic in a firewall or similar (or at least allow the things SLAAC and neighbor discovery need).