Getting rid of the long lines is easy -- go to Mail>Prefs and click on the Composing tab. In the lower section, under Responding, uncheck "Increase Quote Level" which, if checked, indents previous emails and gives you the line. Unchecked will give you no lines and no indents.
As for the full headers, when I Forward, all the headers are there. But when I Reply, they're not. I remember wondering about this a couple of years ago, but then it occurred to me that it actually makes sense. When you're forwarding an email chain, it's generally going to someone outside the chain, so they need the headers to follow the conversation. But where all replies are direct to each other, the single line giving the date, time, and previous writer gives you all that information, and the subject line of the email itself gives the last piece. In other words, it's more efficient to use a single line.
That said, I agree it would be nice to have the option of including a set of headers in a reply as well as in a forward.