Not in order of importance:
1) CONCAT will not properly handle REF arguments that are either REF(variable) that in itself is a REF to another variable or is of the form REF(a;b).
2) REF, CONCAT etc. will not work outside of numoccurs and value tags
3) USEREF does not work
4) EVAL should be more robust
5) EVAL should eventually migrate to a Java math parser like JEPLite (http://jeplite.sourceforge.net/) or JMEP (http://jmep.tigris.org/)
6) Constraints are not implemented
7) Aliases are not implemented and would help clean things up
8) 2D arrays should better show as 2D
9) Easy way to fill in values in an array
10) Easy way to reread a description or a whole set of descriptions
11) A way to reference the k-th element of an array is missing from LCML
12)