My take on things is the single biggest factor is attitude. As a general rule, violinists want to play what's on the sheet as clearly and accurately as possible, while fiddlers want to play around with it. In the past it might have been argued that most violinists had better technique, but i doubt this is true anymore and some things traditional fiddlers do would be hard for a classically trained violinist to do, so technique has to be judged on what it's for. Stylistically, I think the difference has more to do with the bow arm than anything else. Fiddlers don't lift the bow off the strings as much, often press down less on the bow, often slur more. These are all gross generalizations of course, and there are all sorts of grey areas, like jazz, which tends to have more classical-style technique, but little interest in reading anything off a page, cross-over players, etc...