First, a question: any random person on the internet, without being logged in, used to be able to Thumb Up a YouTube video?
If that's true, then I bet it was very easy to spam. Giving it a Thumb Up, without being logged in, would also mean that none of your friends had any idea that you had given it a Thumb Up, and you could never again find a history of things you had Thumbed Up.
And now, to upvote, you need to be logged in...
What website, anywhere on the internet, allows you to upvote without being logged in? Not HackerNews. Not Reddit. Not Facebook.
By knowing who you Follow, Google and YouTube now have a much better way to find information that you care about. If I search on YouTube for "python", it has a chance to know that I mean the programming language, because of who I'm Following.
Also, every single +1 that you get on your YouTube videos now has a real person behind it. Every single +1 is now worth Gold. Every single +1 has a better chance to go viral, because the Followers of that person who clicked +1 have a chance to see it.
If I search on YouTube for something, and I see "Wil Wheaton +1d this," I'm MUCH more likely to want to check it out.