If you're shopping around for a projector, it's easy to get lost in the sea of specs that companies throw at you. While the importance of many features will vary depending on the type of user you are, there are a few details that you should always pay close attention to.
Although projector descriptions will often boast of the different types of resolutions the devices can display, the one you should pay attention to the most carefully is the native resolution. This is the true resolution capability of the projector. While a projector with a VGA native resolution may be able to display an SXGA picture, the conversion process that it brings the image through can sometimes have a negative effect on the quality of what's displayed.
In short: It's a lot easier for a projector with a high native resolution to display a picture with a low native resolution than it is the other way around.
Projector Central goes into more detail about this issue.
Lumens or Brightness
The number of lumens a projector can display will give an indication on how bright it is, and it will also provide some insight into its lamp life. A projector with a high lumen count will take a greater toll on its lamp, and you could end up replacing your lamp far sooner than you would for a projector with a lower lumen count.
The number of lumens you require in a projector will vary depending on how large you want your image to be displayed, and the amount of lighting you will have in the room. The Projector People have a very handy chart to determine how many lumens you require.
Many projectors come with built-in speakers that are capable of filling a room with sound, while others will barely register a tinny echo over the noise of the fan. How much volume you require will again depend on your environment, but you can also verify if the projector has the capability to connect to an external audio source. Pico projectors, for example, often require an external audio source.