Here is such a list http://libreplanet.org/wiki/List_of_software_that_does_not_respect_the_Free_System_Distribution_Guidelines
Most of these packages are removed either because they recommend non-free software, depend on non-free software, are useless without non-free software, or include non-free code.
Trisquel also removes some Ubuntu-specific packages such as branding and Ubuntu One-related packages.
