Hey there! I don't usually post much in the forums, but I saw this here and I couldn't help but give you a reply, because about a year ago I had a little voice of my own saying pretty much the same things yours is. My RP wasn't anything brilliant: I had very little actually planned plot-wise, and I had never GMed before. I was terrified out of my mind that it wasn't going to gather any interest, and that even if it did it would fail miserably. I went through with it though and even if it was a bit of a rough ride, it was successful.
So, my personal opinion of your roleplay (as posted here): I think the idea is great and can develop into an interesting story, but you should have a main plot and an idea of how you want it to end. Are the characters supposed to find out where the powers came from? Are there going to be people abusing or misusing their powers? Do all the people in the city gain powers? If not, how do the people who haven't gained them see the ones who did?
If you base the RP solely on the characters and their interactions people might get bored and stop posting. You should give them freedom to add things and develop their characters, but it's important that the a story moves somewhere and the players feel involved in it even while their characters are out doing their own thing.
This guide is simple and lists all the duties of GMs and RPers alike. I think it will help you better develop your RP and understand what is the role of the GM in an RP.
Hope this has been helpful in some way.