My first request would be to have those SEXPs which don't function (i.e. orders, is-tagged, another one that I forget now) fixed so that they start actually doing what the Help says they do.
Well, I found out why order doesn't work. Even you non-programmers can get this:
int sexp_order(int node)
{
return 0;
}
Is-tagged is redundent and I think was made earlier then has-been-tagged-delay. The code is a lot simpler and doesn't have very good checking for unusual situations. (Like checking to see if a dead/not arrived ship is tagged.) Error will work under debug builds but not under release builds. It calls int3() which does nothing on a release build. Ship-Guardian and Ship-No-Guardian work but not the way you might think. Find the "Most Useless SEXPs" thread in FS Modding for details.
As far as ripping out the SEXPs, that would screw up backwards compatability with regular FS2 missions. If possible, some sort of SEXP set to use functions from some external scripting system might be useful to some people, but I doubt all the mission builders want to learn a whole new scripting language.
Something like the system D3 uses: Regular Dallas scripting with support for scripting via C.