Normally when I'm working on a new project without clearly defined platform requirements, I'll start with most the modern platforms available and then work my way backwards. It speeds up development and causes less grief.

I noticed that as well. If your userbase looks a bit like StackOverflow's or HN's it's probably not such a big deal. If your userbase isn't a bunch of developers, designers or early adopters and looks more like W3counter's then 14% of your potential desktop users (tested IE8, presumably similar on 7) get a loading screen when accessing the try.discourse.org demo. If the apparent limitations to IE9 support are significant, that's a quarter of a potential userbase that can't effectively participate in a community, at least not without writing new code - presumably connected with the infinite scroll feature - to support still-common legacy browsers.

Say what you like about the crappiness of php forum software, but their designs that haven't been updated since about 2001 just work.