While this is a historic time for marriage equality, my fear is that a victory such as this might motivate the opponents of marriage equality into an all out frenzy. Last I checked, 30 states have passed state constitutional amendments prohibiting same-sex marriage, and most of the remaining 20 are in the process of passing them. This victory in New York could become a rallying point for the Moral Majority crowd out there, and lead to not only getting these amendments on ballots across the country, but turning it into a national issue during a Presidential election year.
And before someone comes along and tells me the latest polling saying that most Americans support marriage equality, I think it's a bit misleading. While that might make a good case for general society's acceptance of the LGBT community, that doesn't necessarily correlate with political reality. I suspect that the shifting attitudes in same-sex marriage and other LGBT related issues comes from polling young people, 18-20something and either in or just out of college. If a poll is taken of registered voters about same-sex marriage, I suspect it would be different than what it would be than if you polled the general population.
CORRECTION: I originally wrote that "all" Democrats in the New York state Senate voted for the legalization of same-sex marriage. While I haven't reviewed the exact roll call, reading over various articles from New York media, a handful of Democrats did vote against it.