if there is a yes result, and scotland goes into the EU, then the EU will be tied up in argument for years if not decades, as other regions try for independence. It means the treaties and other stuff, which require unanimous agreement of all member nations, would have to be revised, to a majority agreement. Because having umpteen member nations means umpteen possibilities for vetoes, and for demands for concessions and special treatments. Concessions and special treatments mean why even bother with an EU at all ? what is the point of an EU regulation if 95% of the member nations are exempt from it ?
It means a reduction in social welfare, and less opportunities for people from poorer regions. E.g. look at say, North/South Italy. iirc, Berlusconi or one of his associates wants some bit of n.Italy, i think round Milan, to be independent from south Italy, which has a lot of social problems (immigrants, organised crime, organised criminal immigrants). they want Milan to be independent, so Milan taxes don't get spent on "the wrong kind of people". that makes s.Italy less able to afford to deal with the problems, and as such the EU would have to become involved directly, because having society collapse would be a huge problem. So you'd have a bunch of the small prosperous regions, and they'd complain about having to contribute to the EU, because they see the EU as spending money on "the wrong kind of people". And those regions would have a veto power on EU action. So all those little regions with right wing governments, end up screwing over the entire rest of the EU, and we're looking at a reversal of decades of social progress, and the resultant political violence, which will screw everything up for decades.
the whole point of the EU is that nationalistic sentiment is to be curbed, because europe has had enough of european wars.
for the american posters, the member nation veto power would be like... take the US Senate. Now, imagine that instead of a majority of senators voting for something, it required unanimity, that every single senator had the ability to veto any piece of legislation. that's what the EU has at the moment, kind of.