0. The simplicity of Tcl stems from its simple syntax and processing model
The Endekalogue and the simple resolution and application of [lindex $cmd 0] to [lrange $cmd 1 end] is the fundamental form of Tcl.
1. The power of Tcl stems from its processing model
proc upvar X uplevel Y return is a skeleton which allows you to create and extend any tcl command or proc.
2. The universality of Tcl stems from the fact that values have string representation
It is not precisely true to say everything is a string, but everything which has a value has a string value. So any processing can be performed on strings.
2a. Tcl has a rich set of string operators
This enables X (1. above) to decide what to do in the surrounding processing scope, and Y to effect it.