Ok, so now that I've actually tried it, I'm seeing that including, say,
org.eclipse.platform does not remove any plug-in dependencies. If I clear
the dependencies and click compute (in my feature), I still get things
like org.eclipse.core.runtime, which is included in the
org.eclipse.platform feature.
As far as I can tell, then, the only way to setup the base feature (as of
3.1M7) is to go through this silly process of moving plugins listed in the
dependencies tab of my feature to the Plug-Ins tab, then clicking Compute,
then moving any new dependencies to the Plug-Ins tab, and so on ...