Downzoning limits the height of development; landmarking preserves the look and feel of a community by preventing ugly, non-contextual crap-boxes from being deposited in the nabe.
From Halden's link:
A word of warning to our friends in Carroll Gardens: a lot of the crap that is going up in Greenpoint & Williamsburg is consistent with the contextual zoning that was implemented in 2005 (R6A and R6B, predominantly). Its shorter crap, but its still crap. And other than landmarking, there's not a lot you can do about that.
I strongly suggest reading the entire post. I know that there are people who are against landmarking, and I can respect that. But I am for it, and I will work with my neighbors to make it happen. Interested in helping out? Drop me a line. Vince Favorito has been working tirelessly on landmarking and he can always use more hands.