To all of you that voted for Nazi Germany...you do all realize that the Mongolians made the Germans look like little kiddies? They basically wiped several empires completely off the face of the earth - killing on the order of 80% of the population in the near east and middle east. On a percent of the worlds population the Kahn's made Hitler and Stalin both look like pikers. They invented the scorched earth, destroying all of the irrigation works when they passed because they could live off land that was not irrigated but their enemies could not.
True, but that was early on: the Mongols soon realized it was better to milk a civilian population rather than kill them all and loot the bodies. China under Kublai Khan was a prosperous enough place, and more open to outside ideas than any time since the Tang: I'd argue that given their tolerant attitudes and the immense variety of peoples they ruled over, the Mongol Empire would be _likelier_ to support the sort of flourishing exchange of ideas that might bring an early scientific revolution than a long-surviving Roman one, which probably ends up as a rather intolerant CeasaroPapism, judging from historical trends.
Of course, _how_ you get a long-lasting Mongol Empire is another question: if it unifies because of a common religion among the Mongol/Turkic ruling class, bang, you have intolerance again, if it's Christianity or Islam...perhaps we need a Mongol religion...
Nazis Win WWII, even if it only means a Nazi Europe rather than Nazi-World, probably ends in nuclear WWIII, which will not be good for children or other living things, after the horrendous slaughter of conquered peoples which some members of the Master Race on this board are so quick to shrug off. This is only less than a total disaster if the US manages to pull of a first strike while arsenals are still small enough for civilization to survive.
The US breakup and Commification is clearly meant as a paralell to the OTL Soviet collapse, so I'd assume this is a Communism Wins TL. If they accept the reality of global warming, this might not be much worse than OTL in the medium run - most of the world is probably screwed, but with climate change, new diseases, resource shortages, and the end of a couple decades of "Ponzi Capitalism", the same probably applies to us.
British Empire survives: as an absolute Monarchy? Well, this would seem to require a POD well before the US revolution - and it could be very nasty indeed. (Try Stirling's "Cops and Robbers" in his new short story collection "Ice Iron and Gold", or at
http://www.smstirling.com/ ).
Bigger, longer Black death - well, even if it's as deadly to humans as myxomatosis to rabbits, some will survive. And I doubt it's very biologically likely to be that bad- I imagine agricultural civilization will survive, although a lot of larger states may fall apart and some areas regress back to a virtually "tribal" situation, while nomadic and tranhumant lifestyles expand their range. Civilization will rise again - but it may look fairly different than OTL. Some new religions (or new variants on old ones) may well arise, and given that we've probably knocked out the Renaissance, we've also probably knocked out western science, at least as of 2008 AD.
I'm one of those who think humanity is likely to develop science in the long run, whether in Europe or in the Islamic Lands or the far East (fragmented or reunited China, your choice), but it might be centuries or millenia longer than OTL, so I'd rate this as one of the Bad Ones: drastically lowered population densities combined with a possible fear of cities plus apocalyptic religiosity 'a plenty strike me as bad conditions for the emergence of a class of Natural Philosophers.
Confederate States win - really, as messed up as the world is, an existing CSA probably wouldn't make it much worse, unless you posit a real ConfedWank. (OTOH, a US always looking over its shoulder at the Confeds might be a less effective defense against the spread of totalitarianism than it did 1941-1991. Could go several ways).
Slavery continues - we probably have to have a rather nasty and fundamentally _different_ culture than that which exists OTL. Besides that, it's hard to say...
Spanish inquistion expands? Probably requires a rather more successful counter-reformation and the failure of anything like the Enlighnment to develop, if it's still around in 2008. Even the Saudis don't go for burning people alive in public on holidays. Very bad. Europe consists of a bunch of theocratic absolutist monarchies, and Puritan North America makes Saudi Arabia look like a bunch of jolly bohemians.
Ottomans expand further into Europe? Logistics and the inevitable European counter-reaction means the Ottomans are unlikely to make it much further than OTL - perhaps they take Austria and most of Italy, and squash Muscovy in its cradle, but they're not going to be sacking Holland or looting Paris. And perhaps the exposure to renaissance Italy prevents the Ottomans from intellectual stultification in the 17th and 18th centuries - this world might have a larger "first world" than OTL, with Middle Eastern Muslims full participants in a modern world economy.
I'd say that "continued slavery", if we assume a technologically advanced world may edge out Nazis or the Black Death as the worst - probably some humans survive in the Southern Hemisphere in all but the very worst cases of nuclear war and have a chance of eventually rebuilding, and modernity probably arises at _some_ time in the Black Death world, but the "continued slavery" world has a nasty whiff of "boot in the face, forever" for some parts of humanity, and makes one wonder what such people would do with more advanced technology...
Bruce