What are the physical/aerodynamic implications of designing a prop guard for multirotors?
Aerodynamically, one can say that those prop guards create some kind of Fenestron (Like some tail rotors in helicopters), which will reduce the tip losses of the propeller.

On the other hand, they will increase drag in forward flight, reducing the UAV top horizontal speed. If you take a look at the Aibotix X6 you see a one piece prop guard designed aerodynamically to reduce drag in forward flight.
In the particular case of your image, the black web also decreases the area of air flow into the propeller, certainly reducing its efficiency.

Another prop guards disadvantage is the weight they add. Even though they might be made of lightweight material, they have to have some strength to resist any impact.

To sum up, with a prop guards like those, you should loose top speed and around 2 minutes of flight time.

One last tip: if you want to fly your UAV close to trees, forests or power lines, be careful with the prop guards because you might end with your UAV hanging on a tree branch 10m high... (happened to me :/)