If I have a form that is the main part of a page's content, will assigning focus to the first form field via JavaScript on page load have any negative effects on accessibility?
The short answer is no it doesn't make things inaccessible but it can make it confusing. The longer answer follows. 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.