An obvious reason why the polls were off seems inherently related to the lack of an even distribution of polling machines throughout the state of Ohio (and perhaps other states as well) The more poorly polling machines are distributed, the greater the exit polls will show a discrepency from the beginning to the end.
The mythical polling places in Republicana and Kerrysville...
We'll take two mythical voting precincts: Republicana and Kerrysville.
Republicana, due to the unjust application of voting resources has the more machines (let's say 14 machines with a capacity of 100 votes an hour each) as Kerrysville (10 machines) despite the the fact that Kerrysville has 1.015 the number of potential/intended voters as Republicana. (we'll assume that otherwise, Republicana and Kerrysville are completely equal)
you'll see that in the graph below, the votes in Kerrysville were throttled by the lack of voting machines, hence the flat line as opposed to Republicanas peak, which reflects a more natural distribution of people during the course of the day.
A few references
Renee in Ohios Diary (The inspiration for this Diary)
freepress.org