IMHO, the great advantage of ABZ is that it is an extremely lean and specialized program. It basically just downloads complete collections of whatever files (NZBs) - including, of course, the important par and unrar functions. Personally, I hope it stays this way. I don't want a "Swiss pocket knife" program that does everything, but at the cost of doing everything mediocre at best.
spod, as you point out, for people who know more or less what they're doing (e.g., visit VCDquality or read NFOs) a preview function is irrelevant. Furthermore, as every movie release is packed differently, uses a different video or DVD format, etc., I don't see a practical way to implement a preview function. In addition, there are so many posts that are NOT movies, the potential for unnessecary confusion and errors seems endless to me.