Quote -  Out of curiosity...why in the heck are there different media standards all over the world?  I sent my best friend, who lives in Japan where her husband is stationed, a DVD of home movies and goofy things...and her player won't play it.

DVDs are usually region coded -> http://en.wikipedia.org/wiki/DVD_region_code (this is the most likely cause of the problem).
While regionless DVDs do exist, they're not that common.

Other possibilities include wrong type of disc (+R or RW) and/or disc not finalised (if home recorded).
Some DVRs may have their own formatting which isn't playable by other brands (or even models) of player.