[formulate]

Even if Keith were just average defensively (though he's 30th out of 121 d-men with 1000 minutes played in unblocked shot attempts allowed per 60 minutes of ice time), he's clearly elite offensively and elite in transition, and plays top minutes on a great team. Hard to argue with him (or any of the 3) being nominated. Add in Doughty and Suter (and Gio this year) and there are probably your top 5 in the league over the last few years.

ADD: In terms of hits and blocked shots:

Leading the way in hits are defensive stalwarts Cody Franson, Luke Schenn, Mark Stuart and Ben Lovejoy.
Your blocked shot juggernauts are Andrew MacDonald, Chris Butler and Kris Russell.

While these stats aren't necessarily a negative, it should be noted that making a hit or blocking a shot requires you don't have the puck. This is something that Duncan Keith doesn't do a lot of.
As well, I'll take a guy who is more likely to take the puck away by using body positioning and a good stick, rather than a guy who is less likely to get the puck by making a body check.