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 versus a tclkit.
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.
LV: I don't have access, currently, to being able to run base-tcl or base-tk on any platform other than Solaris. If someone else wants to update this page for the other platforms, be my guest.
Note that even though one gets a base-tcl or base-tk with ActiveTcl that doesn't mean that these executables knows about the local teapot repository of packages. All you have, by default, are the packages mentioned above.