I've been playing a lot of Soraka middle and have really been enjoying it. Your Q does decent damage early game and your E is free and does great damage with Q stacks. Her W is a decent heal and her R can save anyone from across the map, providing not only utility but damage.
Early game, the Q does push the lane, but I've never really had the much of a problem with a gank. I can usually silence the jungler before any CC occurs and if I must, flash out while spamming Q.
I understand that she would probably need an AP top to do the "burst" damage (ie as E is single target and Q only does so much damage), but other than that, why is she not a viable mid?
Also, if I said anything incorrectly, just nicely fix it. <3