Number of Forks leading into Dead Ends or
The more forks you have the more locations you have to get off of the correct path.
Number of Forks along the correct path
If you only have to cross one fork on the ideal path then you only have to get lucky once. The forks work best if they are evenly distributed among the different possible paths.
Length of incorrect paths
A short, linear dead end is only a short distraction. Increasing the complexity of incorrect paths can make them even more of a nuisance.
Path of the ideal solution
If you start on the left and pretty much only ever have to work your way right in a straight line, then the maze will be really pretty easy. For harder mazes the solution should at some points get close to the end, while at some points wind further and further away from the end.
Size of the maze
The bigger the maze the more room you have to work with and the more space you have for longer false paths, dead ends, forks etc.
Shape of the maze
It is my opinion that circular mazes tend to be a little more challenging because the circular paths are a little harder to keep track of than straight paths. Others, however, may have different preferences.