Mobility is FAR too much of crutch and has almost no drawbacks.
There needs to be drawbacks to mobility but most of the super mobile champions have almost no drawbacks to their mobility. Most of them have sustain, most of them have massive damage, most of them can case their mobility without a target (thanks CertainlyT!) and mobility has all of TWO abilities in the ENTIRE game that counters it on two champions that are rarely played.
With how much mobility there is in this game WE NEED A WAY TO COUNTER/PUNISH SAID MOBILITY. Whether it be an item that functions like Poppy's circle and we have to rework that ability on her, or Rylai's causing a grounding affect, or taking sustain away from mobile champions so that poking them actually becomes counterplay to them, IDC. Just give us SOMETHING to make it feel like we have anything more than hoping they misplay themselves as counterplay.