This is very interesting, especially the way curlies are used to make opening/closing delimiters explicit and to extend the inventory of possible delimiters. If I may I would like to suggest adding {|underline|}, {!strikeout!} and {.small caps.}.[^1]
Small caps are somewhat frequent in linguistics (mainly for grammatical tags in interlinear glosses) so their absence is a problem for me, albeit a problem which can be worked around with a span with a class.
I actually like the four space rule, but I guess nothing stops me from just always make sure that a list marker is followed by at least three space characters. Hopefully this will be easier to live with with indented code blocks gone!
[^1]: Incidentally this is the same solution as I used in the home grown lightweight markup which I used before I discovered Pandoc, except there the curlies were always mandatory. I used a Perl hash mapping punctuation characters to LaTeX command names and a simple regex substitution `s/\{([[:punct:]])|[[:punct:]]\}/$1 ? "\\$command{$1}\{" : "}"/eg` except that `[:punct:]` was the list of actually used characters. Those were the days! FWIW I differentiated between {_emph_} for emphasis and {/textit/} for object language.