Should there be circumstances in general conversation (not just quoting someone) where it's not offensive to tell someone that something they like is "kind of gay" (as in wimpy) or that their business proposition is "retarded"? As far as company, I'm referring to the general population excluding people that are offended by anything and everything.
Well, clearly there are. Take this site, for example. The key is to know when and what those situations are, because if you freak that up then it's entirely on you. There are plenty of people who would quite legitimately and reasonably be offended by the use of such terms (for example, gay people or the parents of mentally disabled children), and it's your responsibility to not cause that offense, not theirs to be not offended.
I'm not offended by people on JO referring to something as being gay or retarded but there are circumstances and contexts in which I would find those terms offensive, either by the company I am in at the time or by the way in which the terms are used.