What does "soft video" mean?

That reviewer most likely was referring to the imagery being less than vibrant and/or lacking correct color. That sort of observation could be subjective as we don't know how he determined that the video was soft. A video can look different in say 4 variations. Like from the camera direct to an HDTV via HDMI cable which has little if any signal loss. Or the video can look soft if watched on the camera's LCD screen, or on a computer monitor, on a website, or on a DVD. Each viewing device could even be calibrated wrong. If you found many reviewers posting a soft video issue, then it may be of concern. Keep reading reviews and then formulate your own opinion.