Hi,
Regardless of my last email, I've now committed a minor change to
expl3 repository by which all of \let:NN, \glet:NN, \let_new:NN, and
\glet_new:NN are defined with \def_protected_long_new, and the
variants are plain old \def_new. (No change, then, to \let:NN at all.
Just bringing the others into line.)
The only real change this makes that I'm aware of is that
\let_new:NN \par \foo
will now complain '\par already defined' rather than 'runaway
argument'. I think this is sensible since \let:NN itself does allow
\par.
Let me know if I've missed something :)
Cheers,
Will