Force fields are often air-permeable, which not only allows people to speak and listen through the field, but it also allows oxygen to filter in through the force field and thus allow breathing. However when a force field is used to keep water out while underwater, the force field often appears to have the power to actually extract breathable oxygen from the water around it and remove carbon dioxide from within it. This is sometimes averted, and a non-permeable force field can actually be used as a weapon to choke foes. Similarly, they are usually invisible (or at least translucent) until something pushes against them, which means they let at least one wavelength of light (if not the whole spectrum) pass through unimpeded while still keeping lasers or any other emission that the force field's generator considers "harmful" outside.
There is also the matter of the energy that is distributed over the field. Something that pushes against a force field is exerting pressure, whose energy has to go somewhere. While physical barriers absorb such pressure by spreading its energy across their structure, or by transforming it into potential energy via elastic buffers, a projected, free-standing force field has no anchoring, and often no elasticity, so it must be dealing with distributing the force inflicted upon it in some way. Otherwise, if this energy were simply "done away with," there would be no reason for artificial or magical fields to buckle under assault at all. This often leads to a character suffering a Psychic Nosebleed as a way of showing his or her effort against such force, implying that the force field is braced against their brain. Ouch.