YouTube confirmed its experiments with different Like buttons, but says that it has no plans to replace the system with the +1 button. That begs the question of why it would be testing new buttons if it doesn't want to replace the old ones.
The exact wording of YouTube's response is perhaps important, it said "we are not replacing it with the plus one button." Which isn't to say that it's not replacing it or changing the system, just not with the +1 button.
There's probably more to this than meets the eye. Replacing the Like/Dislike system on YouTube with a simple +1 button would prove a major boost for the +1 button. And since users don't need a Google+ profile to use the +1 button, it will work for everyone.
It would also make sense to have a unified system across Google. There already is a +1 button on YouTube videos, but it's not visible by default. Rather, it's hidden behind the Share button and sits along the Facebook and Twitter share buttons.
But Google wants to have a single look and unify all of its products. YouTube has been rather independent so far, but there are signs that it's losing some of that independence.
In Larry Page's vision last year, Google+ was to be integrated into every product, no exceptions. It's happening with YouTube as well and it doesn't look like the YouTube people are too happy with it.
This could be one reason why YouTube would resist adopting the +1 button entirely. Still, there's probably pressure to adopt more Google+ features, hence the experimentation with the +1/Like hybrid button.
It seems that YouTube is also experimenting with removing the Dislike button. That may seem like a good idea and it's worked great for Facebook. The idea is that, if you enjoy something you click on like, otherwise you move along.
But there's one very important use of the Dislike button, it's a very simple way of filtering out really bad videos, spammy ones and so on, without having to watch through them.
There are trolls everywhere, of course, but good quality videos generally have a much larger number of likes than dislikes, so the dislike button is not a problem for them. If the dislike button were removed, good videos would still have the same numbers of "likes" but so would bad videos. Likes are also much easier to fake and game.