My issue with it is that I click on the Hacker News link, step through the slides and then want to return back to the hacker news discussion to read the comments. Unfortunately, each slide is treated as a new history item and Firefox only remembers a finite amount of them so I then get stuck on the presentation with no way to get back to the site I was on before. Tempted to just inject this into each page I visit since the feature annoys me so much:
So, if you're on a search results page, and click next 20 times and then click back, do you expect to go back to the search form, or the previous results page?
Whilst I agree it'd make sense in that particular context for the back button to take me back a page in the results list, I'd rather the search results page made use of actual links which I can use via my browser to make a new HTTP request for another page of results. I dislike sites which insist on loading primary content with JavaScript, breaking the browser's loading feedback (animated icon, hidden favicon) in the process. These sites usually don't handle failed requests properly, either. If I were to post a form in Firefox to a site which happened to be temporarily down, Firefox would display a message explaining what went wrong and giving me the option to resubmit the entire form as many times as I want.
I also worry about the pushState function being misused to hide URLs in XSS attacks which involve query string parameters. It's clear you can also stop a user from going back from your page too, something I imagine could also be misused.
