Users of the first Google Toolbar will possibly remember the like and dislike voting buttons on it. They were available beside the PageRank Meter but you had to enable them manually. (I don't have a screenshot). I used to like them but they were quickly disabled as they were too open to abuse. But Google has returned, Facebook-esque, with the Google +1 Button. I think this is brilliant and much overdue.
A lot will be made of it being a Facebook copy-cat but that's overrated. Much of Facebook isn't original and Google has tried this in the past. It's also not social networking - Google is an aggregator of data and their star rating system, like/dislike voting buttons and PageRank systems are relative dinosaurs compared to Facebook and the like button.
Here's why I think its so important and where it will have some positive effects for SEO.
Beauty is in the eye of the beholder
Users liking sites brings back appreciation for design, UI and UX. If design is really bad and the User Experience is awful, this will could have a negative effect on a website. Of course, design is impossible for search engines to take into account and its pretty hard for humans given its subjective nature.
This to me is great news for people who value strong design and user experiences and is a big shot in the arm for web designers. But it's how the design affects you, not how you feel about the design (note for site owners) - and so "graphics-free" sites like Craigslist will probably garner many likes.
Up until now, design and UIX was taken into account by indirectly valuing the CTR and link back stats that Google collects.
Can the spam
A massive benefit could be reduced spam counts. Rubbish and empty blogs, posts, press releases, content farms and other spam mass generated by ruthless and shameful blackhat-SEO companies and their careless clients should be hit again by giving a direct vote to users
Bringing Balance to the Force
By giving users a vote on pages - it essentially starts to move to balance out the guesswork by the algorithm. For example, if users keep picking the first few sites that rank but they get less likes than other sites - it makes sense for Google to shake up the mix a bit.
Its unlikely that this will be a heavy weight factor at the start. Because internet representation is still anonymous (and Privacy Campaigners keen to see it that way), the system is open to some abuse, so some form of balance of power to limit it.