My biggest fear for btc in the very long term is not manipulation but the 173k btc the FBI have confiscated from the closing of Silk Rd. I understand Ulbricht has claimed the return of the btc, claiming that as virtual currency they cannot be forfeit as a proceed of crime, but seeing that judges confiscate anything of value in such cases he's on pretty thin ice. I believe these coins will eventually end up in the hands of the US government who will have far too much power - just think, 173k coins all sold on same day. It's a while off (I've no idea when this is coming to court or how the US government usually deal with liquidation of confiscated assets) but this is what we should be worried about.
One question related to those funds is if some of them might belong to people who committed no crimes (or at least have not been convicted of them). I'm have little doubt (in fact, I pretty much know for sure) that there's precedent for the cops just keeping the whole lot but still.