He's really great at the fundamentals of the position. He plays very well positionally, knows where the play is going, and most importantly, takes out his man, leaving the puck to be picked up by a teammate (unlike Pietrangelo, Cole and Shattenkirk, who leave their men too quickly after a hit, and allow them to fight for the puck).
He's more like Jackman, keeping the man out of the play (against the boards or knocked in the opposite direction of the puck), so his own teammate will be the closest, and only one to be able to pick it up.
Of course, you have to know when it is safe to do that, and make a different choice of play if an opponent is nearby (e.g. tie up the puck along with your man on the boards until one of your teammates can get back into the play. Most of The blues' other D-men seem to be afraid to take their man out of the play and leave the puck. In one's own zone, usually the D-man's shift partner is the closest, and able to swoop in and pick it up. Also. as The Blues' forwards usually backcheck well, as soon as they see pressure on one of their D-men, the forward on that side should be skating behind, ready to pick up the puck.
Gunnarsson has really brought some extra solid, consistent play to our defence, more than the erratic Polak, who succumbed often to a strong forecheck, and had a tendency to lose the puck or make inadvisable passes, of fail at clearing attempts. Gunnarsson skates the puck out of the defensive zone himself, or gets a safe, quick pass out to a forward, or sends a safe pass to his defensive partner, or takes his man out, allowing a teammate to pick up the puck.
He also has joined the rush and joined the offence in the offensive zone, driving in on net, pinching in to keep the flurry going, taking decent shots now and then. He's doing just what I thought he'd do to solidify The Blues' defence (when healthy-and integrated into The Blues' system). With Cole and Butler also improving, The Blues look to be a lot less shaky later this season, especially when Backes, Steen, Oshie, Stastny and Berglund start getting back to doing what they can do, and a replacement for Lindström is found.
If The Blues are one point off The NHL lead while playing inconsistently and poor at times, imagine how they'll do when the defence, offence and goaltending are working on all cylindars (Allen has benefitted from a lot more experience-Elliott back) replacement for Lindström - and the rust is off the others.