I've heard it reported many times before that many of the people involved with the show wanted to move on because of the arduous task of completeing an episode everyday. I'm not one to begrudge people for wanting to move on. We all know it was hard work doing this show with everything that was involved. And as I remember it, DC became less involved over time as he branched out to do other things, so his lack of strong guidance might have had something to do with it.
As for the ratings, ratings are only a guestimation, despite what anyone says. The people who produce the Oscars say that the Oscars had like 40 million viewers for this year's telecast, when it actually was likely a much lower average if you count people that tuned in late and those who turned it off at 11 since it lasted into the wee hours of the morning again. So, that being said, the rating average could be somewhere around 35 million or so. There's no way of telling. DS very well could have had 20 million people watching at any given point in time. 8 million could've watched on Monday-Wednesday, and then 4 million on Thursday and Friday, and the average for the week would work out to be 6.4 million viewers--lower than the highest point.
If we're talking about storylines, I can't really truthfully say that any one storyline brought the show down, except for the Leviathans. It's fact that ratings dropped signifcantly during that story because it is a bit out there (what wasn't on DS?) and people didn't really understand what was going on. Personally, I liked the retread of Turn of the Screw with Gerard and Daphne because while the very basic story was the same, everything else was new and different. One of the ghosts actually [spoiler]came to life during this go around[/spoiler] and that was much more interesting than ones that constantly popped up and pointed fingers. Not to mention the fact that Gerard's plan was on a much grander scale, AND we got to watch Collinwood be destroyed. The lead up to this was brilliant, and 1840 itself brought some of my favorite characters:
Gabriel, [spoiler]whose jealousy of Quentin, even in death, would lead him to murder and deciet of his own father in order to get money. Not to mention the secret that he hid for such a long time, which was a huge bomb when revealed.[/spoiler]
Flora, who was delightfully played by Joan Bennett, and perhaps my favorite character of hers, with Judith being right up there too. Flora was so chipper and doodle-brained at times, she was a delight to watch in that role.
Not to mention Gerard whose wheel's were turning at every possible second to get the next step up, and all the other characters who populated this timeline.