Saying NO U is a way of pointing out that the person's comments apply more to himself and is therefore an idiot and a hypocrite. It is considered to be the ultimate trump to any debate or drama on the Internet.
This meme originated on the Something Awful forums where epic flame wars would often erupt. Eventually the arguments became so illogical and the insults so repetitive that often it needed to be pointed out that the OP was being a hypocrite.
NO U is originated back to simpler days when we as children would engage in what were to be the precursors to forum wars. Arguments over common issues would devolve into anarchy as neither party was willing to concede their point. This inevitably led to attacks on character which subsequently devolved into childish name calling which of course would usually finish with NO YOU. A major problem is that it can start an endless loop of NO U. These are known "No U Wars."
As long as there are forums where any asshat with an opinion can post, NO U will always have a home. Short of ignoring idiotic comments, this is a simple and effective way of pointing out that the OP’s comments are in fact more aptly applied to himself than his subject.