I've met many people on either side of the independence debate who seem to regard Gaelic as one of the biggest casualties of the Union.
However, I think it's likely that Gaelic would have declined at a similar pace even if Scotland has remained an independent country forever -- a large reason for the lingering death of Scotland's Celtic language is the depopulation of the countryside, and most Western countries have seen this development, at least to some extent.
On the other hand, I don't think there's any doubt the Scots language wouldn't have been suppressed and dismissed as a mere dialect of English if the Kingdom of Great Britain had never been created. Scots, not Gaelic, would have been the majority language of Scotland today if Edinburgh had remained the capital of an independent country.
After a Yes vote we'll be in a situation similar to the one Norway found itself in after the ties to Denmark were cut as a consequence of the Napoleonic Wars: Lots of people still speak the original national tongue, but they write in the dominant language of the union. In the case of Norway, the written language was Danish, and the remnants of Norwegian were seen as uncultured dialects.
However, in a surprisingly short amount of time, Norway got rid of Danish and created not just one, but two varieties of Norwegian: Bokmål, which is a Norwegianised form of Danish, and Nynorsk, which is based more strongly on Old Norwegian and on the dialects. The two varieties have converged a lot, so even standard Bokmål these days can be pretty different from Danish.
I wonder whether the same could happen in an independent Scotland. Will Scots gradually gain higher status? Will it become more acceptable to write in Scots, or at least to use Scots words when writing in English?
It's impossible to predict exactly what will happen, but I wouldn't be surprised if a hundred years from now, 2014 will be seen as the year when the decline of Scots was reversed.