I'm watching Ken Burns' The Civil War, and he paints a picture of George McClellan as someone who was perfectly prepared to score significant victories against the Confederacy, but simply decided to do nothing, because he did not trust Lincoln's judgement. Burns seems to side against McClellan in this, but I wonder if this is a commonly-held opinion about McClellan or if there are some who support his judgement of Lincoln and his decision not to act.
George McClellan was a "whiz kid" promoted to commander of the Army of the Potomac at the young age of 34. He was superbly trained (at West Point) and trained his men well, but lacked the confidence for serious fighting that comes with experience.
He himself admitted, “It would have been better for me personally had my promotion been delayed a year of more. Perhaps it was a case of too much – too soon obtained at too little cost.” (Leckie, 410)
The question has puzzled historians ever since. It's hard to look anywhere other than McClellan's makeup, which has led to much psychoanalysis of McClellan.
He had immense administrative ability. It's not too much to say that he saved the Union, with his formation of the Army of the Potomac after Bull Run. His Peninsula Campaign came within 4 miles of Richmond.
But it's also true that McClellan was strangely timid when it came time to force the issue. He wasted countless opportunities, seemingly stalled by insubstantial obstacles. He seemed to lack initiative. Plus Lee got completely into his head. It's difficult to imagine Grant stopping for a month at Yorktown, as McClellan did, rather than sweeping over it in his first couple of hours.
Ultimately no one knows. McClellan remains a complicated figure.
One issue was his knowledge of the Confederate Army. Almost all of the good cavalry went with the South, and without scouts he was reduced to relying on Pinkerton for intelligence estimates. I doubt they were actually paid a fixed rate per Confederate soldier reported, but the result would have been about the same.
So, McClellan was advancing, relatively blind, against forces that were reported to be considerably larger than his own. More resolute generals than McClellan have had problems in those circumstances.