Many years ago a former boss of mine (Peter Ford, I believe) told me "Anytime someone says to you 'that won't scale' during a design argument what you should hear is 'Yo Mama!'". [Ed. Note: For my non-American reader(s) "Yo Mama!" is a generic American insult, it is considered very tame and would be acceptable, if said jokingly, in a professional environment.] Lately I've run into another design argument that needs to join the "Yo Mama!" equivalency list – "That's not RESTful!"
Of course both "That won't scale" and "That's not RESTful" can be meaningful statements if they are immediately backed up with detailed analysis that demonstrates how the objectionable proposal doesn't meet some reasonable definition of the cited quality, e.g. scale or RESTfulness. But, oddly enough, I have never run into anyone who says "That's not RESTful" who then starts quoting chapter and verse from Roy's Phd. Thesis. No, generally I find any time someone says "That's not RESTful" what they really mean is "I don't like that design!" However saying "I don't like that design" doesn't win many arguments, so instead you hear "That's not RESTful". Unless you're me, in which case you hear "Yo Mama!"
[N.B. RESTifarians like Mark Baker are, I suspect, fully capable of quoting from Roy's PhD. thesis but I'm not insane enough to argue REST with them. In fact, I generally avoid the term REST because it seems have become more an object of religious devotion than an architectural style. You will usually only hear me talking about noun/verb separation, the power of names and CRUD.]