HD70 Lens Throw and Lens Shift
The HD70 projector's 1.2:1 zoom lens allows a small amount of placement flexibility. To fill a 100 inch diagonal, 16:9 aspect ratio screen, you can place the HD70 (measured from the front of the lens) as close as 11 feet 6 inches and as far back as 13 feet 9 inches (approximately). The Optoma has no adjustable lens shift (as is typical for DLP projectors that sell for under $2500).
The amount of lens offset is significant, so proper placement of the projector is well below the bottom of the screen surface (or an equal amount above - if ceiling mounting). For that same 100" diagonal screen, the HD70's lens should sit about 16" below the screen bottom. (This is an estimate, so don't build your room around it.) the manual is very confusing on this point. I will correct later with more accurate data. All considered this is typical for a low cost DLP projector, in fact very similar to its closest competition the Mitsubishi HD1000U, which also has a 1.2:1 zoom, and a similar offset.
SDE and Rainbow Effect
The HD70 sports a 4X color wheel, so the vast majority of people will not see the occasional rainbows (if you are sensiive - as I slightly am - they normally appear when a bright moving object - like a white light, moves against a very dark background. Even if sensitive you are not likely to see it on a football game as it's all pretty evenly illuminated. The more expensive projectors often sport 5X wheels (even better), but 4X should not be a problem for most.
Screen Door Effect (SDE) is the result of pixel visibility. In this regard, again, the HD70 is a typical DLP projector, with far less pixel visibility than any LCD home theater projectors (with the singular exception of Panasonic LCD models with "SmoothScreen".
As such, most people will be comfortable with pixels only visible in things like white credits on a dark background, or large stationary bright areas. For our theoretical 100" diagonal screen I would recommend about 1.2 times screen width as a minimum distance (where pixels will be barely visible as described above. For those completely pixel phobic - who never want to be able to detect pixels, even in fixed white areas, 1.5x screen width should easily do the trick. In other words, if you like to sit fairly close to the screen, the HD70 projector works well.
HD70 Light Leakage
No significant leakage. If you look at the projector itself you can see light in most of the vents, but not enough escapes to affect viewing.
HD70 Audible Noise Levels
In quiet mode, the HD70 is pretty quiet, claiming 28db. That's a lot noisier than the LCD competition, but then LCD projectors normally have a significant edge in quietness. With ImageAI engaged the projector gets quite a bit noisier. Optoma doesn't publish a spec but I would guess, that, if 28db is accurate for low power, then it's probably about 33db. Those who really want a quiet projector will not be happy with the HD70 in full power or ImageAI mode, so that should be a consideration. My overall impression (not having both projectors here at the same time, is that the similar Mitsubishi HD1000U is 2 to 3 db quieter in full power. When I view in my theater, the projector sits about 3 feet behind me, and I'm sure I noticed the HD70 more than I did the Mitsubishi. For normal movie viewing in my theater, I tend to run all but the brightest projectors in full power mode (or AI on where appropriate).