Hi
I would like to suggest a feature for SLIME taken from Lispworks.
In LW I can type
prompt> first '(1 2 3) 3
prompt> member 42 '((3 2 3) (1 42 33) (42 11 22)) :key #'second ((1 42 33) (42 11 22))
I.e. a line gets parenthesized if it is not already an expression (of course there are cases where things get hairy, but in general I find it useful).
This may even make it more appealing to people who dislike enclosing parentheses.
Cheers -- Marco