I have a little different perspective, and since this has been such a friendly discussion, thought I might add it.
It isn't 121 v. 135, it's culture. Just as every major airline has a company culture that a new hire is expected to assimilate to, each corp. flight department/135 operation has it's own culture. Airline pilots all make effort to fit into the mold, along with the 6000 other pilots at their company. A lot of corp. flight departments have fewer than 20 pilots, and they all know each other. Someone coming from a major undoubtably has sufficient skills, judgement, and whatever else it takes to accomplish our jobs. What they rarely have is the desire to assimilate. Anyone who has flown with an airline guy has heard repeatedly, At Delta (or wherever) we do it like this. Unless this person was hired to introduce an airline culture, that is seldom welcome.
That isn't just limited to airline guys. Anyone coming with a lot of experience with one organization has a difficult time not trying to make their old job like their new job. I stopped even looking at resumes with 12000 hours, whether it be airline, other 135, or whatever. In the interviews, they pretty much tell you what they expect of you, and rarely want to know what they can do to fit in. We haven't hired off the street for several years, and everyone is happier that way. Not that we would never hire a 121/high time/whatever else pilot, it just is hard to find someone to fit a culture from the pilot pool at large.