Disclaimer: This is not a question about which projector to buy, as this kind of question is usually not useful on SE.
I may buy my first projector soon. It will be used primarily for streaming HD videos/movies in a living room, from diverse sources (laptop, BD player, other boxes). It will be mounted to the ceiling (normal middle european height - 220-230cm or so), the maximum area to be filled by the image is about 3m wide, but it is not necessary to actually use that up fully, I believe. Sitting distance is about 3-4m away. It will be used during the day and the night. Obviously I accept a very limited viewing experience during the day.
What are the important technical points I need to look at when deciding between different models? Obviously it needs the correct inputs for whatever players I want to attach to it. Then I will likely have to look hard at the cost of the device vs. the cost and frequency of replacing the bulb. And noise may play a role.
Is there a industry-wide comparable measure for luminosity? Does image quality differ wildly these days or is it basically sorted? What else is important?