R6RS allows  brackets as identical to parenthesis,
with the condition that they must balance. Do we
accept this extension, propose some other use for
brackets, or leave them unspecified?
MzScheme, Gauche, Gambit, Chicken, Bigloo, SISC, Chez, SSCM, VSCM, and the R6RS Schemes support [+ 1 2] => 3.
MIT, Scheme48/scsh, Guile, Kawa, SCM, Scheme 9, Scheme 7, SXM, Chibi don't. Some throw lexical-syntax errors when they read the [, others complain that [+ is not a defined symbol.
I recommend that brackets mean the same as parens. I recommend against Interlisp-style closing of )s with ] and vice versa.
ick. but probably livable. Why can't people just use real editors?
Used to hate the idea, and now I really like them. YMMV.
The WG voted to reject this feature.