While the tone might have been slightly more heavy, this was always a show with some drama. Jim and Pam getting together and the fallout, Ryan as a cokehead, and Michael Scott vs. Dunder Mifflin are examples of this.
I dunno, I just don't remember any of those moments being particularly dramatic. Perhaps you are right about the tone aspect though. This certainly had a much more heavy tone than anything else they've done and certainly felt out of place. You raise an interesting point though, maybe the tone is more the falling point of sitcoms than just drama.