The first thing would be freedom of speech! You know, there are lot of social network which have political judgement for what the user's writing. And I will not have such judgement.
The second thing is the fairness of the reward system. Which is pretty hard to ensure, as it is always hard to define "fairness", so it is one of the most important thing, but also one of the most hard thing. Which need a very long term effort on it.
The third thing would be the stability of the server, I bet everyone have experienced the previous system down situation and have a lot of bad experience on it!