@Thomo makes a good point about how similar it is to sci-fi.
Some things, I would say, does make it slightly different though:
With an alternate history you can include sci-fi elements naturally, since you can decide any outcome that could spark a technological need/developement of some kind, that we do not have today. In some cases, this would be more believable than some sci-fi settings, but that leads me to the next point.
A main difference between sci-fi and alternate history is the kind of "experts" you'll meet criticism from;
- With sci-fi, some people will quickly be sceptical if they feel something doesn't make sense. A lot of 'common' people have substantial knowledge about technology in general on an enthusiast-level.
- With alternate history, your potential critics are a different group of people. Most people have basic knowledge about the history of our species, but of course, some are experts and others have hobby-knowledge.
I'm not sure which is easier to pull off, but I'm tempted to believe that these two groups (generalizing here) have slightly different approaches to the fictional "this is how the world is" -story. I do not know many history-enthusiasts, and therefore I appologize if I'm too influenced by a societal generalization of "such a group of people".
In my mind, history-enthusiasts are "always" looking backwards; Discussing, analizing, researching, theorizing, what-have-you, in general approaching their topics with a "This is how the world was/is, because..." and perhaps - "That's why this happens again here..." and such. These enthusiasst would most likely also have a basic understanding of how people act in/develop/manage/destroy societies. I believe people in this category are generally well read.
In my mind, Tech-enthusiasts, however, are "always" lookign forward, trying to figure out what the next big gadget/tech breakthough is/could be. In the world of tech, "everyone" wants to dream, always wishing for something world-changing to suddenly appear, always expecting some "completely new". You don't hear the phrase "History repeats itself" a lot in the tech-enthusiast-society. Everyone can essentially be "an expert", simply from having a gadget or having held it/tried it.
I'm not saying that history-enthusiasts never look forward. I'm not saying they never dream or think "what if". Nor am I saying that tech-enhtusiasts never look backwards. My point is that the audience will have different ways of looking at the fictive world.
Wether you're writing alternate history or sci-fi, it's a good idea to have a thourougly researched knowledge of:
- "How this could or could work if it existed." (for sci-fi)
- "How have similar events affected societies, and how did they occur" (for alternate history).
Both are of course good to consider, in any of the cases.
However, in general, info-dumps should be avoided. You don't nessecarily need to explain everything to the reader; Knowing it, as the writer, may simply make it easier for you to create an overall authentic world.
Unless you seek a super-critical audience, leave your explantations as simple as possible:
"6 years into WWII, Hitler finally conquerred the US."
"The Carbo-Tear-Fysionater can create small charges of electricity from the air around us."
We don't necessarily care how something workds or how something happened, unless the story gives us a reason to question it - usually with an attempted explanation.