I never noticed it before, but you're right, Odo doesn't do much shape-shifting as the show goes on. I bet it was a budget thing that just eneded up fitting his character arc, as he got more comfortable with this group of solids, he no longer felt the need to or felt comfortable changing shape. But as Laas point out, he's missing out.
(Side note: as a physics teacher, Odo's constant violation of the Law of Conservation of Mass has always bugged me)
The Changeling that impersonated Martok somehow miraculously survived, and he cooked up a convoluted revenge plot on Odo.
As I watched that episode I kept thinking, "Why does this guy seem to familiar. It wasn't until years later, and Memory Alpha, that I figured it out.
See, Kira loves Odo so much that she decides to let him go. That's not really my style of love, I come from the clingy if-you-leave-me-I'll-make-your-life-a-living-hell school of romance, but I appreciate the sentiment.
I'm with you on that one.