Slyvester McCoy had one hell of a task in front of him, he had to try to keep interest going in a show that the fates had deemed unfit to live, and he gave it everything he could. His performance of the Doctor harkened back to a bygone era (I guess two doctors prior is bygone now), as he sort of merged the 1st and 4th Doctors into one being in an ugly ass vest. His performance was totally enjoyable, if not exceedingly dark, and, if he had been hired as the 5th Doctor, I think he would have lasted a LOT longer than he did (as would the public's interest in the show).
His Doctor was different from any of the other's however, it was dark, brooding at times, while being silly and light hearted almost at the same time. The same man would play the spoons in one scene and destroy (what he thought was) the whole of Dalek civilization in the other. He would prat fall, and a few minutes later TALK THE LAST DALEK IN THE UNIVERSE INTO SUICIDE. Dude was dark holmes. He also turned into a vampire for a bit, but for the life of me I still don't know why, and I watched the damn episodes.
His death was silly though, having been shot after being in America for all of 3 seconds by a random group of gang members, because 'Merica.
When discussing the first doctor's run one must separate the first season from the other seasons. When Doctor Who was originally planned, it was to be an educational show (One episode would have Doctor and crew head to the past, to teach history, the next would be in the future to teach science). That never panned out, as the writers were either to lazy to write about factual history, or to creative to be pigeon holed into such a silly idea, so it was dropped. During that first season however, Hartnell played the doctor as a grumpy, thoughtless, selfish old man, a very unpleasant man who you couldn't really connect with, as the main character's were his companions. After the switch, The Doctor loosened up a bit and became much more likable and more grandfathery, and the (true) Doctor was born.
One of my favorite parts about Hartnel's Doctor wasn't really a planed thing; William Hartnel suffered from an undiagnosed case of anterior sclerosis, which made remembering lines *incredibly* difficult. As a result of this, Hartnel would stumble over his lines all the time. The BBC, having no time for re-shoots, apparently, would just let it slide, making the doctor seem seem bumbling and absent minded - a trait which is still used today.