I think the problem with the theme weeks is thus; I have, according to my MAL, seen one-hundred and fourteen anime films or series. I had not seen any of those picked for the theme weeks at the time (bar the first episode of Gurren Lagann) so I didn't have much to talk about. I could have suggested some of my own but it was a case of bad timing as I wasn't around the forums much at the time it was being discussed.
I think the problem, and the reason why discussion can be pretty slow here in general (I think I've said as much before when Aion's been in a mood about how quiet the boards are) is that out of say, twenty fairly regular posters - how many are going to have watched / be watching the same thing at the same time? Occasionally a few people do seem to be watching a series at once which generates a bit of discussion (Higurashi, Haruhi, Eden of the East are some I can think of) but it doesn't happen often.
I think if the theme week concept is to work, it needs to be more of a "anime of the month club" where people are encouraged to watch and talk about the show, perhaps for the first time, with those who have seen it writing why they think it's good to begin with, and then those who are interested watch and report back. A week just isn't long enough for people to become interested and buy / watch it.