How Bright or Dark is your room?
How Many People will be in the room?
This will dictate the image size you go for. The more people in the room the larger the image will be, so a brighter projector is recommended.
How much light is in the room?
For the best image from your projector you will need a dark room. This however is not always a viable and suitable option. Projectors are often used in environments where note taking is required. examples are Training Rooms, Boardrooms and Offices. If you are looking to purchase a projector where note-taking is a must you should look at brighter projectors.
Projector Screen or Wall?
Walls are often used as a projector screen. If you intend to use a wall as your screen you will require a brighter projector as walls do not reflect the light as well as purpose built projector screens.