Here's the thing about downvotes; almost no one downvotes. And another thing: some people downvote anything. Don't believe me? Check out this chart from when Youtube stopped using 5 star ratings:
For the most part people are much, much more willing to note what they like, not what they dislike, at least in simple rating systems (fully written reviews tend to be pickier about dishing out the love). They've even gone with the trend (and saved some clutter) by omitting the "dislike" label (note that if they kept the Dislike table, the dislike button may have been slightly larger):
I do think Youtube tried to demphasize thumbs-down, but this doesn't necessarily tell you much. Virtually every video on youtube with a non-trivial amount of votes has downvotes. The fact that it was downvoted at all is almost irrelevant; downvotes only become relevant there when a large portion (not number in absolute terms) of votes are downvotes. And the bar gets that point across very well.
Basically the Youtube bar doesn't need to throw in your face "this video sucks look it has 10 dislikes WTF! Oh also it has 28734 likes but who cares"; in most cases, all you really care about are likes. In cases where dislikes start to matter (large dislike:like ratio) downvotes become naturally visible. Actually Stack Exchange does that as well; downvotes are "hidden" either behind click or until a post is actually in the negatives. While visually very different, the structures afford the same "preference" to positive votes.
Small numbers of dislikes shouldn't be highly visible. They aren't significant. As a youtube producer myself, the single downvotes random videos get are almost entirely useless. It's only when the ratio of votes sways unusually negative that users should really take note; that's what both Youtube and Stack Exchange are doing (and many other sites, such as Facebook and Twitter which only allow positive "votes").