Impressive work in so short a time. I'm relieved that you guys are this responsive to the community. You've already addressed one of my personal concerns and hopefully the other (sort by date) soon. Here's some more feedback.
I think the defaults should be "all" (not story) and "forever" (not past week). I searched for something that I knew existed and when it didn't come up, my first thought was that the search was broken. Better to show everything at first and let the user decide how to narrow it down if they want to.
I think you'd be better off if the design looked like HN itself, especially the comment rendering. Not that what you've got is bad, but the user is almost always going to go from HN to search and back to HN. Any context switch is already a speed bump, and visual changes, which take time and mental energy to process, add to that jolt. Anything you can to do minimize these cognitive hurdles will serve the #1 goal, which is to get the reader the info they're looking for with minimum overhead. In this respect, the old HN Search is more usable, precisely for being unoriginal.
Thus, if I were you, I would drop the thumbnail pictures of the stories (it's cool that you can do it, but they don't really add anything and are distracting); would not include the story info with each coment (rather, I would do just like HN does and have a bit of text that says "| on: the-story-title-linked-here"), would make the text rendering look much like HN's, and would follow HN's lead in having a text- and information-density-centric design.
I'm not saying that HN's design is the global optimum (though it is better than most attempts to improve on it), but rather that HN search is an extension of HN and therefore not the place to innovate on its design. You're in a counterintuitive position for a startup with this project, since calling too much attention to yourself in this context is bad. You want to be unobtrusive and have the thing just work; the HN community is smart enough to figure out who you are from that and like you better for it. (That said, you shouldn't obliterate yourselves. For example, I like the visual cues that say "Page 1 of 10, got 237 results in 3 ms" and "1,244,896 stories and 5,289,181 comments indexed". They are unobtrusive and impressive.)
Lastly, a bug in Chrome: if I search for something, scroll to the bottom, click on "about", then hit the back button, the original results page freezes (i.e. refuses to scroll).