YouTube is experiencing some issues at the moment and is intermittently unavailable for users. It's not a complete outage, but some users report seeing a blank page and being unable to use the site. The mobile apps seem to be affected as well, so it's not a site-only problem.
Google has been having problems for the past 24 hours, intensifying some 12 hours ago. This would seem to suggest a likely culprit, Hurricane Sandy.
As the monster storm hit the eastern US shore, it caused flooding and destruction, but it also led to power outages throughout the regions.
Data centers in the area have been affected. While all data centers do come with power generators, so power failures would not affect them, the power outages could affect network connections.
Other issues such as damage aren't as easily managed too. The Huffington Post, Buzzfeed and Gawker were all hit by problems at the New York data center that hosts them all. Gawker is still down, The Huffington Post is running a minimal website with some updates on the conditions.
While Google does have data centers around the world, quite a few are located in the region affected by the hurricane. If one or more of them were knocked offline, it would put a huge strain on the others, explaining the difficulties in handling traffic.