For usability it can be useful to autofocus on something like the search input form on a search page.
Where the primary purpose of a page is to search or add content, it can be appropriate to assume that is what you want to do and help you get there as quickly as possible.
In Drupal 8 we're struggling with this here: https://drupal.org/node/2096347
Note that this isn't just a Drupal issue, but certainly something that we're looking at.
There are some great descriptions:
Unfortunately, there are no guidelines for when to use HTML5's autofocus properly.
As @Jared says "Will your users know that there going to a page with a form, and does there need to be any descriptive text you should read before filling out the form? I'm a screen reader user and it can be annoying having focused put in random fields. It's clear why your focus winds up in the Google search box so that doesn't bother me. If my focus were automatically placed in the answer edit field every time I viewed a question on Stackoverflow I would be annoyed since I'd have to force my screen reader to navigate away from the form field and to the top of the page."
But is that enough?
We want to look at usability and accessibility of autofocus. The page needs to be able to meet WCAG 2.0 AA requirements.