"You used to talk more than me" is my first inclination, and it's colloquially relevant. My sense is that you need the word "do" or "did" or "would" etc. if you use an "I".
But it looks like it's an ongoing controversy: http://grammar.quickanddirtytips.com/than-I-versus-than-me.aspx
You are correct "I" is the subject even without adding "did"... You finished the eliptical sentence by writing the "did". DL penalized you because you expressed something that the sentence conveys but which exists in our head. More precisely the thought conveyed is " You used to talk more than I talked"....or "you used to talk more than I did." Please read my several comments below.
The question isn't whether you have any of the words you list in the sentence, but what the subject in the sentence is. Just because many (native) speakers make this mistake, does not mean that I should be forced to make this mistake as well. If DL decides to accept this it should not mark a response as false where that translation is not selected.
The issue is that if enough native speakers make a "mistake" in a natural language, it isn't one. We didn't get English on golden tablets from a god on some mountain. Its "rules" are observed trends among speakers and little more.
At the very least it should also accept "...I" though.
Some things might be accepted, but just because native speakers make mistakes, that doesn't mean the mistake should be made correct. I hear people say "I seen" all the time, just to name one of the worst offensives. More people than not confuse your and you're. Simple English. Their errors shouldn't mean we should deem such atrocities correct.
I think "You used to talk more than I did" will work fine if you want to use 2 subjects. But other than that "You used to talk more than me" is perfectly right, because it answers the question, "More than whom?", and not "More than who?". This is how I've understood it. Correct me if I'm wrong.
I vs. me debate aside (a good text on it can be found here: http://www.quickanddirtytips.com/education/grammar/than-i-versus-than-me ), "Usted" is the only really expressed subject of this sentence, as you can see from "hablaba". Perceived continuation "more than I talked" would be another clause, with "I" , but not "you", as its subject.
Charlie, "Usted habla más que yo." would work for your translation. ...... Why not do what I just did? Go to (the freely downloadable), Spanishdict, and enter "Usted hablaba." The entire DL sentence will pop up !! Then click on "more translations"! And there it is :) [Of course, you can simply google: Spanish Dict]
I is correct. me is colloqual and perhaps should be accepted. I read a lot of US crime novels etc and I find that Americans are more grammatical than natives. I is not the object of the verb to be. When I was at school a long time ago I was taught that the verb "to be" takes a complement, not an object. which means that the "do" is understood".