First and most important of all!
x = y = z
This should always always always be two separate assignments. First, y = z. Then x = y, which is then equivalent to x = z. My intuition won't allow for anything else, and I think it would be the same way without my experience in C-esque languages. So, the first order of business is that any method, new or old, that changes this behavior is right out.
x = (y = z)
Ordinarily I would rant just as vicariously for this case. However, this is AHK, and in AHK I'm used to parentheses denoting an expression. Therefore, since I took the trouble to explicitly say "Hey, this is an expression," AHK should say "Okay," and do an expressiony thing: compare. If that's what you wanted to do anyway, you wouldn't write it looking like consecutive assignments... that would be incredibly confusing, both to code and to read.
What it all boils down to is this. When it's obviously an assignment - which almost always means it's the leftmost operator in an otherwise ambiguous expression - then it should assign. When it's in a cleary denoted expression (parens, if statements), it should compare.
In addition, It'd be nice if we could eliminate non-delimited string assignments with '=', and make them all expression assignments. For instance, this should assign the contents of variable "b" to variable "a", not the string "b".
a = b
Finally, I think that the distinction between '=' and '==' is very clear and intuitive. I never understood the whole issue about bugs and such because I never had a problem with them myself. However, I also think that, for someone who has never glimpsed a C textbook but needs a powerful automation tool, it would be a bit vague, or at least awkward. It makes sense to say:
If a = b
Humans are smart enough to figure things like this out from context. I think the old syntax should remain, with the only change being from strings to expressions as mentioned earlier.
Anywho, that's my two (or three) cents. If I made any grave errors of the logical variety, please call to attention my stupidity. Thank you.