The cond macro, for example. It had all those gratuitous parentheses [...] I wouldn't be surprised if someone had already discovered the Arc trick of collapsing if and cond into one operator

That's done among others by metalua. In AST view, an if-then-elseif-end statement is seen as:

      `If{ cond1, block1,
           cond2, block2,
           ...
           condn, blockn,
           default_block }

Qi also did a great job of cleaning up a lispy syntax. Too bad it hasn't got macros...