Implement hyperexponentiation/tetration without the use of '^'
J, score is 7 (12 chars - 5 points for avoiding multiplication)
+/@$/@$~/@$~
usage:
   4 +/@$/@$~/@$~ 3
1.34078e154
t=.+/@$/@$~/@$~  NB. define a function
   4 t 3
1.34078e154
   2 t 2
4

Just few nested folds:

Using multiplication it would be */@$~/@$~
Using power it would be ^/@$~ where $~ creates array, / is a fold function.