How involved was he in the planning and execution of the operation, and was his involvement good or a bad for its success?
Hitler, at the least, established the target objectives and the date for the operation. He ordered that all preparations for the offensive were to be completed by the 15 of May, 1941 (Messenger, pg.71). He also noted that the Red Army was to be the main target of operations, rather than any geographic center (Glantz & House, pg. 31).
Hitler also made 2 key executive decisions during the execution:
He delayed the operation by 1 month in order to invade the Balkans. One of the new Yugoslavian government's first acts was to make nice with the Soviets and talk with Britain about joining the allies. Hitler was very upset by this and turned his army south east for Operation Marita instead, postponing Barbarossa (Messenger, pg. 72).
Hitler Issued "Directive 33", which ordered the majority of Armored forces from Army Group Center on the 19th of July to support attacks in the North and South. This was criticized by some German commanders for delaying the advance on Moscow and increasing wear and tear on those units (Glantz & House, pg. 75).
Now whether these moves were good or bad is a matter of opinion. I think delaying Barbarossa by a month was a mistake. Yugoslavia had to be dealt with to protect the Axis flank for the mission, but Germany needed all the time it could get until the winter took it's toll. He probably should have delayed it for a year and started in spring 1942.
As for the second order, Hitler probably did push his forces too far. I'm a fan of this quote: "By late october, the Wehrmacht and the Red Army resembled two punch-drunk boxers, staying precariously on their feet but rapidly losing the power to hurt each other" (Glantz & House, pg. 81-82). In the end, trying to take down Russia in one blow seems like it was too much for Germany. Perhaps the whole mission was doomed from the start for being too ambitious.
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