Two major blunders...
Not finishing off England as a combatant, leaving an allied "standing stone" in the west.
Attacking the Soviets in fall, (I seem to remember reading somewhere that the generals wanted a spring offensive, but logistical problems kept delaying it).
I wonder if they could have have waited until next spring though, as Stalins purge of trained and/or experienced officers was at its highest and the red army in complete disarray. They might have have been able to ship out new indoctrinated junior officers and ncos at least the following spring.
He (Hitler) had a strange habit of being decisive in both opportune and inopportune moments. His ruthlessness prevented a total collapse the first winter, but the same ruthlessness when changing strategy on the fly, overruling army officers, also managed to spread the war effort thin against a numerically superior enemy, rather than decisively go for targets within reach. Leningrad and Moscow in particular.
Oh yeah, and the assumption that the lines on the map actually meant roads.