Outbreeding Depression is just plain depressing.

Opposite of Hybrid Vigour (sometimes), when you have two individuals of different populations, subspecies, breeds or species mate, Outbreeding Depression might occur. Unlike Hybrid Vigour, there's much less to explain here, but unlike it, Outbreeding Depression is less common in nature.

As generations of animals roll on, and nature does it's thing with picking the winners to have more babies than the losers (or even the slightly less winners), animals start to become very good at what they do, and often very adapted to where they do it. Their DNA has been winnowed down to genes that make them outstanding whatevers1. Arabian Gazelles are great at being arabian gazelles, and Turkish Gazelles are fantastic at being gazelles in Turkey. But what makes a good Caribou over here in North America doesn't make a good wild-reindeer over in Eurasia...

Take this case from Templeton (1986):

"...when the Tatra Mountain ibex (Capra ibex ibex) in Czechoslovakia became extinct through overhunting, ibex were successfully transplanted from nearby Austria (Greig 1979). However, some years later, bezoars (C. i. aegagrus) from Turkey and the Nubian ibex (C. i. nubuana) from Sinai were added to the Tatra herd. The resulting fertile hybrids rutted in early fall instead of the Winter (as the native ibex did), and the kids of the hybrids were born in February - the coldest month of the year. As a consequence, the entire population went extinct (Greig 1979)."

There's a number of species we're worried about outbreeding depression. Scottish Wildcats interbred with domestic felines, heavily burdening its genes with domestic copies. The same is true for Sand Cat in Saudi Arabia, and the Ethopian Wolf. Closer to home, there's evidence that domestic reindeer from our reindeer herding past (and lesser reindeer herding present) have introgressed into caribou herds - that is, left reindeer genes where they ought not be. No one's sure how extensive it is, and what, if any, consequences it has for North American caribou.

1 Though sometimes, natural selection traps them in a nasty situation (such as the Irish Elk, whose antlers were simply too big).

Greig, J.C. 1979. Principles of genetic conservation in relation to wildlife management in southern Africa. S. African Journal of Wildlife 9:57-78.
Templeton, A.R. 1986. Coadaptation and outbreeding depression. 105-116 in M.E. Soulé (ed.) Conservation Biology: The Science of Scarcity and Diversity. Sinauer Assoc., Sunderland, MA.