Anything to do with the US civil war
Granted, it's an American board but seriously. Especially the obsession with Confederate victories annoy me - I'm so sorry, but Forrest winning a skirmish in 1862 or the confederacy introducing more Gatling guns in 1861 probably won't change anything. Get over it.
US- or Brit-wanks
It's not the timelines as much as the implications. Guess what country is always a war-torn hell-hole led by incompetent dictators just waiting to be conquered by the US? Mexico! Subhumans the lot of them.
I'd love to see a timeline where the US does indeed conquer Mexico, only to get bogged down in an endless insurrection they will eventually lose - parallel to the U.K. in Northern Ireland.
And speaking of the U.K. they will always be the premier superpower and will always have an unbeatable navy. The French, republic or empire is a matter of flavor, will always end up a political mess, the Germans will either fail to unite or eventually lose a war to the English (bonus points for the English winning a land war with Germany/Prussia by using their navy somehow).
And of course the colonies will eventually realize how awesome the British Empire was all along and begin enjoying being colonized. I've never seen more colonial apologists than the ones arguing for the British Empire. Sure, they would violently oppress people and play ethnic groups out against each other in ways that have led to gruesome civil wars and atrocities. But they build a really good railway system in India, and you can't put a price (in human lives) on that!