The main difference is the U-shaped masking on the D2R that blocks out unpredictable light in certain directions. Also, the base has different notches. Name of base is P32-d2 for D2S and P32-d3 for D2R. D2R was developed so that Mercedes could still use their reflector based headlights in the mid 1990s. They did not want use projectors like BMW. A glare box inside the headlight could have been used to accomplish a straight line, but a mask painted on the bulb was cheaper to make and had better precision. Other (near) luxury cars (Lexus, Infinity and Acura) followed Mercedes and equipped their cars with D2R bulbs in reflector based headlamps. Using a D2R in anything else than a OEM HID headlight designed for the D2R bulb makes no sense. It will only reduce output. Some HID kits comes with D2R bulbs. Some amateur kit designers will even claim that D2R genrally reduces glare in a retrofit. This is nonsense. The D2R bulb masking is around 3-4 a'clock and 8-9 o'clock when bulb is in right position. Those are not the (only) sectors that creates glare.
As an example, here is the data on Philips 2 versions:
D2S: 3200lm, 4250K, 91lm/W, 35W
D2R: 2800lm, 4150K, 80lm/W, 35W