"Except" in general doesn't behave predictably, but we'll do our best in explaining this.
Except for and except are both just different ways of saying with the exception of. Easy peasy.
But wait—there's more.
"Except" by itself can be used as both a preposition and a conjunction. "Except for" is only used as a complex preposition-type thing.
Example 1
I like every single ice cream flavor except for rocky road.
OR
I like every single ice cream flavor except rocky road.
To be honest, we think both these examples are wrong (who doesn't like rocky road ice cream?), but grammatically, they're both correct. Except for and except both mean without or were it not for and are both used as prepositions here (with "rocky road" as the object).
Example 2
I like everything about you except that you have an evil twin.
OR
I like everything about you except for that you have an evil twin.
In this case, only the first one is grammatically correct. Since "except for" functions as a preposition, it needs an object. The phrase "that you have an evil twin" can't be used as an object because it's a clause, which is why the second sentence is grammatically incorrect.