No copying of parameters, just pass the address. Parameter can be changed in subroutine. The calling program has complete control as to whether the changed parameter values should be copied back at the return from the subroutine 6. Implicit declaration of varibles - easy to mispell a variable name and the error is difficult to find. Goto is used in many different ways - makes code hard to read and understand.
Labels are numeric - no meaningful so makes code harder to understand.
Weak typing - harder for compiler to catch programmer errors. Comments occupy whole line - can't comment at the end of a line. No nesting of data structures - more difficult to create data structures needed. Character type doesn't exist or at least not a full fledged character type. There are other valid answers - these are ones I thought of.