I know I've said this before, but the infinite scroll always catches me out.
I often brush on the trackpad to get to the bottom of the page. If there isn't a bottom of the page, it just freaks me out. It makes me think something's broken. Maybe I'm just an outlier though ;)
My hunch is that most of the feedback is from technicaly inclined people and most of them are expressing the 'coolness' of the infinite scroll rather than a coginitive-usability perspective. Maybe try doing a feedback round with non-technical people, possibly with something like usertesting.org. Also, the infinite scroll is a pretty old paradigm and Google would have definitely run it through its user tests, but htey obviously dont find a lot of value in it.
That being said, wrt to fonts and sizes, I wonder if you like any arrangement after tweaking the settings? http://duckduckgo.com/settings.html
On 2, I get way more positive feedback on this than negative, perhaps 10/1, though I do agree there are issues around the edges.