HDR means different things to different developers. To some, it's just doing blooms, but if they're doing HDR with tone-mapping then it's fairly significant. The short description is that it simulates how your eyes respond to bright or dark lights, when looking at a bright light other objects gradually get darker, and with low light the whole scene would get brighter. The likely reason they had HDR before and now don't is that to get optimal speeds it breaks FSAA (a frustrating hardware limitation that takes too long to explain here), which is I suspect why some time before release they said they weren't supporting FSAA. In the end if they're doing it right then it can make a big impact on the atmosphere presented.