You are correct, there was a thread like this a few weeks back, but I think it is a valid topic for discussion and as the season evolves, people's thinking on this evolves as well.
My list is based on the question of who I personally am most disappointed in based on my own expectations from last year and previous years, and not necessarily who I think are the players who have performed worst this year.
My list:
1. Merriweather
2. Light
2. Butler
4. Banta-Cain
5. Ty Warren
I put Merriweather first because going into the year I thought he had progressed really well last year and thought he may become a star player. Instead I think he has regressed quite a bit and this has caused problems for the defense as a whole.
I put Light in there before Butler because to me the poor performance of the running game has started to catch up with the offense. It is somewhat unfair to single him out, it would be more accurate for me to list the offensive line as a group here, but since this is a listing of specific players, I had to go with Light because I think both his run and pass blocking have been spotty. I am also of the opinion that with Mankins out, Light's performance has been affected by having a lesser-skilled player at left guard, so hopefully with Mankins back in there, Light can prove me wrong.
I think the Butler listing is pretty widely accepted by folks who post on this board. Listing him here is not a 100% terrible thing, since it means we still have some hope for him. Lots of players go through a "sophmore slump," so hopefully a light will click on for him (better be soon, though, just ask Wheatley).
I put Banta-Cain here because he showed last year he has the ability at times, but it hasn't been there at all this year. I can't think of a single big play the guy has made all year. I feel like he plays this spot by default, there just isn't anyone better than him right now on the roster. They need a major talent upgrade here, I think.
Warren shows up here because I think his absence has really impacted the defense both on and off the field. Belichick's reaction to his season-ending injury spoke volumes to me about what BB expected.