As someone who uses RS-485 on a daily basis, perhaps I can be of some assistance.
The 750 ohm resistors are used as a fail-safe to ensure that the A and B transmission lines are held at a known voltage when there is no communication. The RS-485 specification calls for a minimum differential of 0.2V when the comms are idle, so your 750 ohm resistors are fine, even if you have them duplicated on both ends of the bus. This does cause a current leak, however, since the networks are the same on both the 5V system and the 3.3V system - there will always be current flowing from one to the other through the 485 lines which is no good. In cases like this the networks should either be tailored to their power supplies so that the voltage across the 121 ohm termination resistor is the same on both ends, or only exist on one end of the bus. However, this fail-safe is redundant in your application anyway because the MAX3485E already has a built-in failsafe feature. Therefore, your failsafe resistors are doing nothing but drawing an extra 3mA from your 5V supply and an extra 2mA from your 3.3V supply (roughly speaking at least -- the fail-safe networks existing on both ends of the bus with different voltage levels makes it a bit more complicated than this).
In regards to the common ground wire, while it is not strictly necessary since RS-485 is differential, it is usually still a good idea. This is usually done to ensure there is not a drastic difference in potential between two isolated systems which may cause the receiver inputs to see a voltage that is outside if its specified range. For example, if we designate the driver ground as 0V, the remote receiver may have a ground sitting at, say, -20V. The transmitted signal is at 5V with respect to the driver supply, but it's at 25V with respect to the ground of the receiver IC. This puts it outside of the 485 spec (and thus, the capability of the chips) of -7 to +12 volts, and that can damage the receiver. A common ground wire ensures that the grounds of each system are within a safe range to prevent over-voltage of the receiver inputs.
NOTE: I just re-read the comments and it has been mentioned that the driver and receiver are already on the same ground. Therefore the black wire in the schematic should indeed be removed, as KISS states. The black wire should only be connected if the transmitter and receiver chips are on different grounds.
As to the original question, yes - talking to a 3.3V receiver from a 5V driver can cause damage to the receiver IC (speaking from experience). I recommend incorporating level-shifters or, at the very least, a series 10 ohm resistor on the A and B lines to protect the receiver from the higher voltage driver.