How do you tell people their work is of poor quality?

[Hototo]

By telling them their work quality sucks. When you start to think of their reaction you are implying that they are not only a "bad worker" but an "incapable of regulating own emotions worker" which I would personally always consider offensive.

Personally I prefer "Your work sucks here*points at*, do it better."

to "two longwinded pages of text where somewhere I imply that this area there might be a tad bit poor".

Saves everyone time and you can get to the heart of the issue quicker for all parties involved. If they make an issue of your straightforwardness, its their problem. You can direct them to a anger management / general counselor if they react.