I don't like the idea of the browser visiting pages before I've told it to.
It's not rendering it (I would image at least), I don't see this as a security concern, do you?
I see this as annoying when I'm reviewing my server logs.
Actually, it is rendering it, in a hidden tab. As far as the site will know, you visited. Last login times get updated, web stats record hits, stuff gets marked as read, etc.
So scripts execute too. Hm, that is a bit surprising. Well, I trust Chrome so much that I don't block scripts and I go to any site that pops up as a google search without any hesitation so I suppose I don't mind.
I could care little about privacy concern, but I would rather wait a couple of seconds than waste bandwidth downloading pages that I would most likely never look at. Also from web developer point of view, I would like to see how any given site loads under standard use case anyway.
Google is only preloading the top result when it is confident the top result is what you are looking for. If Google does a good enough job then only a small % of preloads will go unviewed.