1. Good at logistics and training, bad at everything else. As a commander, he defeated himself over and over again, either by assuming that the enemy had greater numbers than they did or through indecision as to when to commit resources. Even when given the enemy’s battle plan in advance, he only managed a draw.
2. No, he would have been a terrible president. McClellan was utterly opposed to emancipation, freedmen serving in the U.S military, and civil rights for African-Americans. Most importantly, in 1864 his own party’s platform called for a cease-fire and negotiated peace, while McClellan ran on an impossible platform of continuing the war to restore the Union but not interfering with slavery, even as the war was destroying slavery.