Rainbow Effect and Color Wheels
DLP projectors rely on a single DLP "chip" and bounce white light against it. Color images are created by timing the light against a spinning color filter wheel that typically have red green and blue segments (and sometimes white). Without getting into the intricacies, some people, due to the color wheel, see flashes of red, green and blue, referred to as the Rainbow Effect. Early DLP projectors (about 10 years ago had 1x (one times) wheels. Today's projectors use faster wheels to reduce/eliminate the effect. Most business projectors use 2x wheels while almost all home theater projectors today use 4x or 5x wheels. (A few older entry level HT projectors still use 2x.) Having faster speed wheels on home theater projectors is due to the rainbow effect being more visible on motion video than still (business type) images.
The Optoma HD72 projector uses a 4X wheel, a little slower than some, like the BenQ PE7700 with its 5X wheel. As a result more people will notice (and possibly be bothered by) the rainbow effect.
Problem is, no one seems to know what percentage of the population has an issue with the effect on a 4x projector or a 5x projector. If I had to guess (a wild one), I would say 5x DLP projectors are only an issue for 1-4% of the population, and I don't know if that number increases by 25% or 100% for a 4x color wheel. Pure guesswork.
Let's say that dealers report very, very few returns of DLP home theater projectors due to the effect, but if it bothers you, bingo: Get an LCD projector.
This is the usual tradeoff. Some people see the rainbow effect, and therefore buy LCD, while many people who like to sit closer to their screens for more immersion (the really BIG screen feel), prefer DLP because pixels are less visible (screen door effect). The good news - most buyers are happy with either technology.
In summary, there's a small possibility that you might notice the rainbow effect with an HD72, but not on one of the other DLP's with a 5x wheel. I would think its more likely that if you see it on one, you'll probably also see it on the higher speed, but notice it a little less. Again - my opinion based on no good numbers.
I certainly wouldn't avoid the HD72 because of the rainbow effect unless you already know you are susceptible to it based on viewing other projectors, because very few will notice it. If the HD72 is your first DLP projector though, power it up, feed it a movie and check it out right away (projecting on your wall will be fine), and be aware of your dealers return policy.
The projector as noted elsewhere, claims 27db noise levels. Since most will run the projector with AI on, that should be your noise level. Other projectors are quieter, in their best modes (low power), and some are down as low as 22db. Still 27db is a very respectable spec. It was only 2 years ago that most home theater projectors were in the low 30 db range. Unless you are sitting right next to the projector, and what you are watching, is virtually silent, you aren't likely to notice any noise, let alone be bothered by it. (Yes, if you are listening for it, intentionally, you will hear it if the room is quiet, of course you'll likely hear 22db also).
Lens Throw and Lens Offset
To fill a 100" diagonal 16:9 screen, the front of the lens of the HD72 can be placed as close as 11.48 feet, or as far back as 13.81feet. (Based on Optoma provided specs.)
You can find a chart on the HD72 brochure on the Optoma website for seeing what size screen will work from any given distance. There is also a lens throw calculator that I haven't played with.
The HD72 has a lot of lens offset, which is the projector's fixed lens shift. The projector, if ceiling mounted, will have its center of lens, above the top of your screen surface. Conversely, if you place it on a table, the center of lens needs to be below the bottom of the screen surface. The amount of this offset is more than found on most projectors so you may have a problem if you are converting, say, a basement where you only have 7.5 feet tot he ceiling, and a fairly large lens, because the projector will be well above the top of the screen. If you use a flush type mount + the distance to the center of the lens, you will probably still be at least 7-8 inches from the ceiling.
I should note that most projectors with lens shift, will let you place the projector almost anywhere vertically between the top and bottom of the screen, some will even allow offsets that will allow the projector to go significantly above (or below) the screen surface). Competing LCD projectors usually have lens shift with lots of range. This is one of the weaknesses of DLP projectors in this price range.
DLP projectors like the Optoma HD72 and the BenQ PE7700, as well as the forthcoming InFocus IN76, lack lens shift. The offset can vary a lot. Whereas the Optoma mounts well above top of screen, the BenQ, by comparison, has a 0 degree offset, which puts its center of lens even with top of the screen. It's all just a matter of what will work for you, in your room.
According to the manual the lens offset is 6.52 degrees to the top of the image, and that assumes you are using the full 768 vertical pixels, whereas, I believe most people will use a 16:9 screen and only 720 pixels of height. Depending on the zoom setting, for a 100" screen, I measured projecting the 100" diagonal from 13.8 ft - (the greatest distance for that sized screen) the offset was approximately 15.3 inches from center of lens to bottom of the screen. BUT that would be for the 16:10 image. So add an additional 1.6 inches to the bottom of a 720 line image. End result, you are looking at having the center of lens 16.9 inches below (bottom of screen) or 16.9 inches above (top of screen) for a 100" screen at max distance of 13.8 feet. (Please note, these were quick measurements - do not rely on the them for precision.)
And the projector manual is NOT helpful. It provides calculations only for 16:10 screens, and for you to actually do those calculations, you need to know your tangents. I have already sent an email to their product manager for the HD72 recommending that they simplify and post on their site.