There are a number of smaller considerations that may also have an impact on your purchase.
Lamp life: Lamp life refers to the length of time your average bulb will last before it needs replacing. A company will list a projected lamp life, with the average at around 1500 hours. Although this sounds quite lengthy any small bump in the wrong place or even storage in unfavourable working conditions can lower that expectancy. You want to be as careful as you can, and factor the quoted lamp life into your decision, as bulbs cost upwards of a few hundred dollars. Some companies offer warranties on their bulbs if they break before a certain amount of usage (around the 200-hour mark), so keep an eye out for those manufacturers.
Remote Control: Most projectors will come with a remote control, but this is not always guaranteed. Nothing is more irritating than having to fiddle with your PC while you're in the middle of a presentation so check for this added feature before purchase. Many projectors also support remote mouse control, which is a handy but not essential feature.
Fan Noise: This is a concern mainly with home theatre projectors as the last thing you want to hear when watching a movie is the projector fans. However, even in a business environment unwanted noise can unsettle even the most experienced public speaker. If your projector is roof mounted, noise levels will be less critical but if the projector is positioned on a table just feet away from your audience, it may be of concern. Many manufacturers will list a decibel level for fan noise which is a good indication of what you can expect.
Heat levels: The fans in a projector are constantly cooling the bulb by pushing hot air out of the projector through one or more vents. This means that using a projector will always raise the temperature of the room it is being used in. If your projector is mounted on the ceiling, this will be less noticeable but if it is not, a small room can heat up fairly quickly and make your audience rather uncomfortable. With home theatre projectors, if you have a smaller lounge room, you may want to consider a flat panel TV over a projector because most home theatre projectors will produce quite a bit of heat. Alternatively you may want to consider mounting the projector on the ceiling to avoid turning your lounge room into a sauna. Heat levels aren't as much a problem in business presentations as the projectors are usually much smaller and produce less heat, nevertheless research the heat levels because this is not a hard and fast rule.
Keystone correction: This is a feature that helps correct the image when the projector isn't sitting straight in front of your screen. It's achieved by compressing part of the image to make it appear squarer. There is a more advanced version of this on some high-end projectors, which actually alters the position of the lens to help combat the positioning of the projector which may be worth looking into if you plan on buying a high quality device.
Anti-theft: Several high-end projectors also have anti-theft enabled to discourage thieves. Operators are required to enter a PIN upon powering up. Alarm systems can also be implemented for permanent installations.