I don't really care for Valla mid, personally. She has vault which is great but she can easily die if the gankers have any form of CC. She also doesn't have the greatest gank potential around. She can land some good damage with a simple W/Q combo but has no real way of securing a kill. If she has to vault aggro to secure the kill she generally has to eat some tower damage which kill almost kill her or she exposes herself to vaulting into the mysterious unknown with her low HP pool. She is a competent wave clearer with her W build but I find her best laning elsewhere so she doesn't have to worry about ganks from multiple directions.
Usually I prefer my mids to be tanky, ungankable and have gank potential. Like I'd prefer to have an Arthas, Kerrigan, Zeratul, Muradin in that lane to a Valla. But it all depends on your comp and the enemy comp as well. And which lane people take doesn't matter in this game since solo CS means nothing.