Great voice, amazing goal call, his PBP flows remarkably well in a sport that demands it, and his voice has the natural air of gravitas needed in any elite announcer while also containing the sheer manic energy required to heighten an NHL playoff game even beyond its inherent insanity. When I think of a loose puck around the net in playoff OT, Doc is the voice that I hear in my head.
And maybe it just sticks out even more in comparison to booth mate Eddie O, but it's also worth mentioning that Doc might be the best in the business (all sports) at being totally impartial in his call of any given matchup. He was all about the action, and on top of that you never had to stop and wonder as you do with some prominent basketball announcers, for example, why the hell the announcers are spending 20 minutes talking about a bad restaurant experience or something of equal value.