^ I have to respectfully disagree with your assessment of Echo as a character during S1 and your assertion that S1 is largely comprised of standalone episodes. As I pointed out in my reviews, Ghost, The Target, Stage Fright, and Gray Hour aren't truly standalones; they just seem like they are. As for Echo, she is not by any means the blank slate you think she is even though she has no innate personality any more, as each of the first four eps tells us or demonstrates things about her character that carry over from ep to ep.
You're entitled to your opinion, of course, but I got the impression from the original post that you've only seen maybe four or five episodes of the series in total. Revisit my analysis after completing the series, and you'll see that we don't really get to know Echo until much later.
When Dollhouse first aired, much of the criticism aimed at Dushku was that she seemed emotionless in the role of Echo. Sierra and Victor came off as much stronger than she did, in part because their characters were in love.