Everybody's reacting to this as if this imposed some new limitation or newly-expanded definitions of mental illness. According to the article, that's not what Cornyn is proposing: "People who have been legally ruled 'mentally defective' or been committed to mental institutions are already barred from buying firearms. But states are not required to send those records to the FBI-run federal database, leaving it uneven." [emphasis added] It appears to me that Cornyn's bill is simply trying to improve the validity of the existing NICS system, using already-existing criteria, by increasing the amount of data provided to it. No expanded exclusions are being proposed, and no legal recourse is being taken away.
Real-world example: Seung-hui Cho (the VA Tech shooter) was able to legally purchase his gun because they didn't have the records of his previous commitment in NICS.
Add to this the fact that gun control proponents don't like the proposed bill, and I'd say this is actually an improvement over the status quo.