* Deadlifts incorporate the arms.. Squats do not
* You can cheat fairly easily with the squat (lots of people do by reducing the depth of the squat) but it is pretty hard to cheat at a deadlift.
Deadlift works the spinal erectors, hamstrings, glutes, forearms, traps, biceps, quads, core and even the deltoids are involved in the lift. The only way the squat could compete with the deadlift would be if you were comparing it the overhead squat but in order to do this variation of the work one would need to develop coordination, balance and flexibility in order to use any significant amount of weight. The deadlift is just so straight forward.. you can either lift it or your cant. Very little room to cheat, works more muscle groups and is fairly easy to do without having to practice over and over and over again with a broom stick (like you would if practicing for a OH squat)
or in otherwords DEADLIFT FTW!!!!