Steem (And SteemIt) are off to a bright start in terms of becoming important players in the social media market, but there's a couple of major problems that need to be tackled:
Bot abuse / spam
Zero transaction fees make spamming way too easy, and far too profitable. While there are some measures being taken to reduce fake signups, there's a significant number of "dummy accounts" posting to the blockchain without doing anything useful. These abuses need to be slowed/stopped in order for the platform to be more appealing to new users. This is also creating major bandwidth issues, which will likely be exacerbated as more users get on board with the platform.
Accessibility
SteemIt requires a fair amount of research before a user can understand its various currencies, payout models and rules. More needs to be done to simplify the platform for newbies who may not be familiar with cryptocurrencies. Presently the platform is great for content creators looking to make profit, but it doesn't serve viewers / subscribers well unless they are also content creators.
I have no doubt in my mind that Steem will do well and see incredible growth over the coming years, but there are some issues that are hurting user retention and could definitely use more attention.