Titanium grid fins for unlimited re-uses.
New landing legs with the ability to retract instead of having to be removed after landing. These legs will also be black instead of white.
Changes to the turbopumps to prevent turbine wheel microfractures. This was never considered a risk by SpaceX but NASA asked SpaceX to fix the issue and from all reports they have.
Replace paint with thermal protection barrier coating for the purposes of re-use.
Improved heat shielding around the engines to improve re-u****lity.
The octoweb (structure that holds the engines) will be bolted instead of welded, to reduce time for inspection/repair/refurbishment and to allow easy change from F9 to FH side booster.
The interstage will be black instead of white.
Upgraded fairing, Fairing 2.0, which is very slightly larger and has changes to allow for recovery and re-use.
SpaceX's upgraded COPVs (dubbed COPV 2.0) will fly on Block V. This is an upgrade to further reduce the potential for an incident like Amos-6.
Another improvement in thrust for the Merlin 1D engines (roughly 10%).
The rocket will be man-rated, meaning it will be certified to carry crew. NASA has set the bar at 7 successful flights of the rocket for certification.
Upgrades to valves, seals, and many other parts to allow for many re-uses.
Improved flight control, angle-of-attack, and control authority which should allow for landings with less fuel (and therefore the ability to land after lofting heavier payloads).
Edit: Clarified entry about paint and turbopumps. Added flight control entry.