80% is the exit criteria for the milestone. If we don't make it thrgouh the sprint (even though we do plan the time up front), we add it through the stabilization. We might take an exception for particular component or feature, but we open Pri 1 item for the next milestone.
During coding, code coverage is measured automatically on the daily build and the report is sent to the whole team. Anything that falls under 70% is yellow, under 50% is red. We don't fail the build currently, but we have a plan to add this in the next milestone.
Not sure what the dev happines has to do with unit testing. Devs are hired to build quality product and there should be a process to enforce minimum quality and way to measure it. If somebody is not happy about the process, they are free to suggest another way of validating their code, before it is integrated with the rest of the components.
Btw, we measure code coverage on automated scenario tests as well. Thus, we have three unmbers - unit, scenario and combined.