According to the blog, "We have made a few changes based on issues we saw during internal testing. It will be interesting to see what people think of those changes. No hints on what the changes are, we want you to play with fresh eyes."
The blog encourages beta-testers to give their impressions at the L4D2 Steam forums.
Beta testing has appearently been helpful, as the blog noted, "Thanks to your testing and feedback we have gotten more data on Blood Harvest. This caused us to change the bridge collapse sequence a little bit to give Infected more area to spawn above the bridge and to fix an exploit in The Train Station."