Sentence:  Hey, watch it! I'll be the first to admit that RSTS is old, generally ugly, and very stupid about lots of things, but it IS <WSD>hackable</WSD>. UNIX is equally, if not more, ugly, stupid, etc., and is also <WSD>hackable</WSD>. The difference is that in order to get UNIX to do ANYTHING even REMOTELY USEFUL it MUST be hacked. Of course, if you like case significance and an operating system designed around the same mindless philosophy, go ahead and use stock UNIX. For now, on PDP-11s, I'll stick to hacking RSTS. 
Meaning of the word marked with <WSD> tags:  That lends itself to hacking (technical tinkering and modification); moddable.