A base-tcl interpreter is a stand alone, Tcl only, interpreter which includes the following package names:
So there are a number of differences in terms of what packages one will find by default in a base-tcl.
The base-tcl and base-tk, just like the tclsh that comes as part of ActiveTcl, include an array which provides build information.
Another difference is that itcl is missing. That means there will be some starkits which won't run with the base-tcl alone.