>>8
I'd say implementing a lisp isn't particularly simple at all and theres a reason mainly post grads make them, the parser is easy though. Implementing a lisp entails that it supports some type of macro system and perhaps closures, more importantly function definitions, but the biggest hurdle I think is that since Lisp doesn't have statements, structures and divisions but just forms everything has to fit together in the big tao of the language, the semantics can't be band-aided for this and that statement, everything has to fit.
Implementing BASIC is simpler just by the expectations of what it will be capable of, its syntax isn't particularly hard and can be parsed a line at a time (Altair BASIC simply tokenized words as they were entered by keystroke using a simple FSM, and executed them just by reading the tokens from left to right until the EOL token with a subsequent address to jump to, at run time), and semantics can be rough and stupid but they'll be isolated to every statement.
Also, Forth is arguably simpler for a statement-less language as the evaluation model is simpler.