10. Failing to bring it up to the present.
Nonsense. That tells you what kind of story you can write, i.e. it must be modern. Screw that; write in whatever period interests you. If modern, yes, that does give you a lot of cool timeline to play with.
9. Not recognizing that some historical developments were probably inevitable.
Variable. More precisely, you need to understand that some historical developments need a really big hammer to kill. I spent a substantial portion of junior high figuring out how to prevent the American invasion, with some help from my father who is a history teacher. I wound up obliterating most of the Eurasian population with plagues and wars.
8. Ignoring historical factors that were important at the time, even if they aren't important to your story.
Valid. Everything is connected. Butterfly wings can blow your story off-course in ways you'd never imagine. Ideally, if you're writing allohistory but not a historian, find one to be your first-reader.
7. Not accounting for even the most obvious ripples from one big change.
Valid. The most important question in this genre is "... and THEN what?"
6. Concentrating too much on the one changed event, instead of all the events that led up to it.
Variable. It depends on what kind of story you want to tell, which is YOUR CHOICE. You can lead up to the big change, explaining why it veers off. You can focus just on the time of the change, which is probably a good idea in shorter works. You can start with the change and then follow the ramifications downstream as it changes other things. Or you can do all of that, preferably in a novel or series of related stories/poems/whatever.
5. Mixing up urban legends with actual history.
Half true. If you do this accidentally, it is a mistake that will probably kill your story laughably.
However, there's no rule against doing it on purpose, as long as you know which ingredients are which. This is speculative fiction. You can throw everything in a blender and lean on "frappe" if you want to. Just understand that sometimes you'll get V8 and sometimes you'll get sludge.
4. Assuming that nothing will change besides your one big alteration — or that everything will.
Usually valid. Very little happens at any extreme of any spectrum. If that's where your story is, be prepared to work extra hard to explain it.
3. Making the story go where you want it to go, instead of where your altered history will support.
Valid. Any version of railroading tends to wind up with your train of plot wrecked in a cornfield, its wheels spinning pathetically in midair.
2. Explaining too much.
Variable, leaning toward invalid. This is a matter of taste, both in amount and in method. Explain as much as you want, because some readers love it and indeed come to this genre for just that. But you have to make it entertaining, and there's a finite amount of sausage that will fit into that sack. Pack thoughtfully. Also, if you're in love with your research because you found amazingly awesome things? Don't be afraid to write it up as such and post it as bonus content. Some people really, really love that -- fans and other authors alike.
1. Forgetting to tell a good story.
Valid across all fiction, and indeed, all entertainment. Boredom is a universal flaw.
Wow, this list did way better than average. I only had to toss out one rule completely. Most were either valid or partly valid.