It feels unnatural that it is not the first button. I also just noticed today that there is a reply icon in the time-line. Did not know. Maybe a more obvious way to know this? Because I would use this instead of scrolling all the way down to the reply button.
Edit: I might look strange because it is left oriented and the reply button is somewhere in the middle.
Edit: I made a printscreen with the reply button on the left:
Personally I find it easier to hit as it is, since my mouse cursor tends to spend most of the forum browsing time on the right side of the screen anyways. This is the side scroll bars typically reside too, after all.