Now, Atwood wants to change the shape of the venerable Triangle, and add a fourth dimension - Quality. If you are up against a fixed budget, with a fixed development team, somethings gotta give, and that thing is typically quality; unit testing and some system testing may take place, but acceptance testing is cursory, technical documentation a pipe dream (to the delight of the techs), and volume testing happens with that first round of lucky users (oh, beta, right ...)
I disagree: Quality is a Feature, and project planners and sponsors must knowingly accept the risk when testing and documentation is sacrificed to make a deadline.
This is best illustrated when working the project plan and the unruly gantt that stretches past the target date. A fully loaded plan will correctly have time set aside for documentation, a full suite of testing, plus training, conversion, and "burn-in time" - some span of time where project members are on-call, watching the systems performance, providing focused follow-on training where needed. When doing a roll-out to multiple departments / locations in the business, we also like to put in calendar "buffers" between major process changes, so the folks on the line / in the trenches don't get blown away with changes every week.
Of course, when the resource leveling hits, and the end date stretches to a full quarter beyond the target ... well, those big stretches of lag time look all too easy to cut out. It takes a strong project manager - or, more often, an experienced business sponsor whose been burned before - to fight for those elemnts to remain in the plan. Success is never about the lines of code that made it into production - it's about delivering the business results that you paid for.
The correct Feature(s) to remove are the Functionality bits - cut the brilliant menu options, the alternative views, the fancy reports, but do not skimp on the Quality stuff. If you lay out your project correctly, folks will see that simplifying the Functionality / Feature set of the software can also cut down the Testing, Training, and Documentation portions of the plan.
The super-simple PM one-liner for the Iron Triangle is ...
Better, Cheaper, Faster - Pick Two
Better means more bells and whistles, but infers bells and whistles that aren't cracked.