Speaker Design Checklist. What should I watch out for?

You have only touched the subject of box alignment. Typically step 1.
Good point. I didn't think of that as a 'surprise' topic that I would need to discover, but some people may find it helpful on the list.


In a home hifi power handling is rarely an issue and given that there is no standard for specifying power handling you can only compare across a single manufacturer's data.
I know I at least take note of the power handling, even if its not comparative between brands. I've toasted a few subwoofers and I wouldn't want to design large speakers with a 12" thats only 80w. Same for tweeters. I saw a planar I liked recently but it was only 20w handling. Probably not a good match to a 500w bottom end.



Overboard,
You listed 'what do guru think of my design' in your list. I think it is inadapted.
The great thing to present a design in a place like this one is this is open to a group of people, all with their own perspective about things.
It helps to make educated choice to have pro and cons of the compromise to be made.
A Guru usually have definitive answer to this ( justified or not) and usually won't take the time to discuss other opposed view.
I agree that everyone's perspective is valid. I supposed 'experts' would be a better term for me to have used. I was trying to get across that it's worth seeing what the more experienced designers have to say about the design, in case anything was missed.