Alternate history is a fascinating genre wherein authors write parallel world wherein history changed and diverged from what really happened. Consider the analogy that the past is another country while alternate history is from a different dimension. Ideally, alternate histories can make you see the real history of our world in a whole new way and make you realize that events that seem like they were inevitable.
Here are the worst storytelling mistakes in the alternate history genre:
1. Forgetting to tell a 'good' story
One of the biggest mortal sin in writing alternate history is taking for granted the art of good storytelling. Even if you create interesting characters, an alternate history will just be a story without a soul if readers are not compelled to read/ There has to be a point to all this stuff, beyond just the fascinating "what if" question.
2. In every story, there should be a sense of direction
When you're trying to tell a story, make sure that you try to entertain and provoke some thought. And when it comes down to it, you can break almost any rule in the service of a good story. As Harry Turtledove would say "making the story go where you want it to go, regardless of whether they change you've made can plausibly take you there."
3. Confusing urban legend with actual history
Make sure that you know what you're writing about because most of us think that we know the history, more than we actually do. We tend to rely on our general understanding of the past when dealing with alternate history.
4. Ignoring key historical factors that were important at the time
A large part when writing alternate history is making judgments about which historical points to pursue. Bear in mind that if you ignore something that was important to the people at the time then you risk throwing some people out of the story.
5. Forgetting to bring it up to the present.
If you don't bring your alternate history up to the reader's present timeline then you leave out half the fun.
6. Some historical developments were probably inevitable
Often, we think of history in terms of a single person who did something heroic and historic — like, Columbus sailed across the Atlantic and started the European age of exploration. The big breakthrough that allowed Europeans to sail the world was a greater knowledge of deep-ocean currents and wind patterns. Once you know the winds blow consistently from East to West off the coast of Northwest Africa, and from East to West further North, it's fairly easy to sail the Atlantic. So if Columbus hadn't sailed to the New World from Western Europe, someone else would have fairly soon after.
7. Not accounting for even the most obvious ripples from one big change
You can't account for all of the ripples from one point of departure because those ripples will have ripples, and so on. But you can pay attention to things that almost happened in real history because they might well have happened, if things were different.
8. Focused too much on one changed event instead of all the events that led up to it
Just as authors sometimes fail to consider the obvious "ripple effects" that might result from one major change, they often act as though a major change comes out of nowhere. Every historical event has a bunch of causes that lead up to it, so if you want to make a major alteration seem plausible, you have to tweak the factors, so that the changed event appears inevitable in retrospect. The real reasons for the change might be ten years earlier or even a hundred years.
9. Assuming that nothing will change besides one big alteration
Unless the change is so major that the world is unrecognizable afterwards, some things will still stay the same.
10. Too much explaining
The biggest problem in the alternate history genre is when an author shows off too much information and complexity and assumes that the reader will be as fascinated with the details as they themselves were. Determining the pertinent details of the time-shift and then integrating them organically is a serious challenge.