Such as?
https://minnie.tuhs.org/pipermail/tuhs/2021-November/024695....
But a couple interesting things I remember hearing (though I'm not sure if they were in the linked page) were passing alternate return address labels to functions, and resumable exceptions (although Lisp and Ruby do have this).
Also, not that unique, but that no keywords are reserved, and so you can have variables named "if" and such.
It`s arguably a good idea to treat keywords as reserved, in order to avoid confusion and to write faster compilers.
No-one should (be able to) write confusing code like:
if if = then then then = else else else = end end if
Out of curiosity, how did you deal with the fact that for a top-down parser, 1 token of lookahead isn't enough to know if, say, you're dealing with an if-statement (e.g. "if x > 0 then ...") or an assignment or procedure call using a variable named "if" (e.g. "if = 1", "if(x, y, z)")?
I feel like the best solution would be an LR parser that reduced to an if statement only when seeing "if <expr> then" (and I saw you mentioned LALR and LL parsers somehow in your documentation), but you could also maybe use a backtracking top down parser.
I realize could read the source code listing, but I think it would be easier just to ask, and I might misunderstand it.
https://www.tuhs.org/pipermail/tuhs/2021-November/024695.htm...
However the integration in the language of those features made them frequently more convenient and I consider the pthreads API available in UNIX/Linux as considerably inferior to the thread handling that PL/I already had in 1965 (e.g. already then PL/I had an equivalent of WaitForMultipleEvents that was better than anything in pthreads).
Even if the C language has evolved from BCPL, most of its evolution has consisted in taking various features either from PL/I or from Algol 68. The C language has introduced very few innovations, i.e. only the "continue" statement and its more general (but more verbose and cumbersome) syntax for the "for" statement. Therefore many features of C and of its descendants come from PL/I, e.g. "struct", ".", "->", "malloc" and "free", signals and many others.
Also PL/I allowed aggregates, i.e. arrays and structures, in many contexts where most modern languages accept only scalar variables, requiring verbose loops for handling aggregates (or the definition of custom object classes, i.e. the implementation of some library for handling them).