I wouldn't recommend flip flops. Too much walking and remember that WDW has inclines and declines (e.g. AK) and not all pavement is smooth (i.e. ongoing construction and concrete work). But, Birkenstock or Teeva branded sandals seem popular but I'm not sure those are actual flip flops.
I always recommend to folks asking for footwear tips to wear closed toed sandals and/or tennis shoes. Also, waterproof sandals are great on days there is a strong chance of rain.
Last month, I wore (and switched between) Keen Newport H2 (waterproof sandals) and Sketchers Go Walk with a goga mat bottom (tennis shoes). Never had any problems with blisters or foot/ankle pain.