Now now it's not fair to generalize moy.
Yandle has had a very strong year, no doubt, and is likely the Norris front-runner at this point in the season, but I don't think he's quite at the Hart level. He has scored a ton of points anchoring Pheonix's powerplay, but he plays sparingly on the PK and as much as I don't like the statistic, his +/- isn't great. If you think about the defensemen who have won the Hart in the expansion era and the seasons they had, it's really hard to put Yandle in the category of Chris Pronger and Bobby Orr.
The only reason I personally chose Zetterberg (right now) over Thomas is because he's been more consistently good, although Thomas has had stretches that were better than some of Z's. Thomas' last month was just average (4-3, 2.74, .906) while Zetterberg has been remarkably consistent scoring at or above a point a game in every month. Now don't get me wrong they're both fantastic candidates as are Stamkos and D. Sedin, I'm simply justifying my choice. I should also point out that I've seen Zetterberg and Thomas alot more than Sedin or Stamkos this season.
An interesting note: if Daniel wins it, it will be the first time in over 40+ years teammates win the Hart in consecutive years (last to do it were Phil Esposito and Bobby Orr in 69 and 70)