There are alot that haven't been solved.
As was mentioned, scalability and ease of use.
And lets be honest, finding a non-speculative use for the blockchain.
Scalability is not a problem, at least not one to be solved or fixed, and definitely a problem all alts wish they had. Look at the blockchains that face scaling issues. Bitcoin, foremost. The very fact it has a scaling issue (which, by the way, actually already has its fix at least for the foreseeable future) points to the fact that it has non-speculative use, which is why scaling is the envy of all alts.
The internet has always been catching up in the scaling issue, and has had far more than its share of ppl saying it would break because it can never scale. But look, here we are. Using Bitcoin over the internet. And both are far from being broken.