Long lines, and lines in general. You won't be able to escape long lines in the summer time. As a child I would have loved to go to Disney World any time of year. As an adult, I don't want to go in the summer time unless there is no other option, or I'm saving money by going at that time. From the time you get there lines everywhere.
The second thing is the weather. More specifically the rain. I think most people factor in the sun and heat in the summer time in Disney World, but they don't put as much thought into the rain. Imagine if you you plan a Disney World vacation in advance and it rains the entire time.