Ok, I asked so I could use these awesome photos in an answer... I confess.
The material has changed from Aluminium with an ablative paint to bare titanium. The specific shape, size, and mount points have changed as well.
Let's start with a nice shot of the Mod 3 design on a Falcon 1.1 Full Thrust.
You can see the size, shape, and design differences in these side by side photos, or as commonly known rocket porn. (Admit it, you just come here for the articles).
Showing some of the structural differences highlighted. You can see that two support columns within the fin are no longer needed. The new fins are a bit bigger. They are unpainted titanium. The mount mechanism uses the same connectors but a different design on the fin components.
Now for a profile shot, look at the teeth on that baby... Betcha they got some bite.
Elon tweeted after the landing: "New titanium grid fins worked even better than expected. Should be capable of an indefinite number of flights with no service."
The previous fins were clearly ablating hard, and getting quite hot during previous landings. Thus these new ones should make reuse one step easier.
I saw a comment that the camera lens on the descent cam did not get covered in goop as usual, and wondered if it was the absence of the ablative paint; in fact in previous recoveries, it was not atmosphere (read: water) getting on the lens cover, but rather the older-model grid fins shedding material. In other words, all boosters with the new titanium fins should be less vulnerable to ablation and thus the camera feed should remain intact.