The Ruby programming language is a poster-child for potential ambiguities that seem simple - allowing conditional before and after for example. Essentially, all the programming languages that are "human like" wind-up like this, with the sort of ambiguities that people are comfortable with in natural language. This has a cost in terms of exact expression but a lot of users think of this as being "easy on them".

Oppositely, languages without grammatical ambiguities often seem irritatingly verbose to use - new users dislike lisp's parenthesis proliferation and personally find Scala irritatingly verbose.

The complexity of compiler-writing approaches as limiting factor to language complexity probably depending what someone is familar with. Some people can spit out annotated YACC grammar pretty easily whereas my head swimming looking at the stuff. I can produce a recursive descent parser from a grammar pretty easily however.