Thanks for the info.
After some investigation, it seems that Media Center will only recognize a VIDEO_TS directory as a single movie when it can find a VIDEO_TS.IFO file in it. I scan from a parent directory down to import my DVDs. Unfortunately, I have ripped a number of DVDs with the main movie only (for space reasons) option. This gives me the IFO and VOB files for the main movie but does not contain the VIDEO_TS.IFO and VOB files.
I tried ripping the VIDEO_TS.IFO file to add to a previously ripped DVD. MC recognizes the directory now. However, when I try to play it, it gives some sort of DVD volume not found error and won't play.
I had to re-rip the VIDEO_TS.IFO, VIDEO_TS.VOB, and the IFO and VOB files for the root DVD menu in order for MC to play the DVD. So, at a minimum, that is what your VIDEO_TS folder must contain before MC will play it.
It's unfortunate that MC can't play it with just the main movie (IFO and VOBs) files ripped like VLC can.