Having just moved my WordPress blog to OpenShift, it’s amazing how many settings apparently I had tweaked and customized on my previous site. I just followed a Google search link to one of my own posts and got a 404, and realized that the link format to my posts had changed.
I used to have links like /year/month/day/postname and now only URLs that looked like ?p=number. The first format is one of the Permalink formats. You can customize this in Settings/Permalinks. More info here.