Voltage and current - Basically because you will run your motor continuously, you will want to run your motor at or below its nameplate rating. Too high of a current means your motor will heat up too much (see below). Higher current also increase the current density in your brushes which will shorten life. Too high of voltage would mean your motor is rotating faster than typical and could reduce bearing life. At higher speeds your brush pressure may not be optimized, either, so you could reduce brush life.
Temperature - Most standard motors are designed for a maximum ambient operating temperature of 40 °C. This temperature takes into account the temperature rise that will result from running your motor at its rated temperature. If your ambient temperature goes above 40 °C (or if you run your motor above its nameplate rating) your motor may getting hotter than it should. If the motor gets too hot, it may affect the life of the insulation, the life of the bearings, or the life of the brushes. If insulation temp is an issue, you can go to a higher class insulation (for example, from Class B to Class F or from Class F to Class H). If bearing temp is an issue, you can look at getting higher temperature grease in your bearings. The temp of your brushes can cause a change in the friction between brush and commutator. You can get different grade brushes with different coefficients of friction that can help you with this. Low temperatures can also affect life. For example, your bearing grease has a lower temp limit as well and you can specify a low temp grease if necessary.
Moisture - If your motor is outdoors or needs to be washed down, then your motor should be sealed. Basically you want to prevent corrosion on metal parts. You can get sealed bearings, water-proof wire exits, o-rings between end bells and housings, etc. to prevent leaking. High humidity can actually cause brushes to have a shorter life, too.
UV - Some plastics and rubbers will degrade in sunlight, so if the motor will see direct exposure to UV, you will want to make sure your materials are compatible. This may affect lead wires, grommets, brush holders, etc.