1. Trending by unmonitored software would be dangerous. But a person choosing items for the FlyerTalk home page could make good use of Helpful count to identify candidates.
I suppose you're right w.r.t helping manual curation; it couldn't hurt, at any rate.
2. Changing thread display according to Helpful ratings was once something I favored. As you say, it carries gaming danger. I no longer favor this complication. It also sounds hard to implement.
I think it might be helpful on a very narrow handful of the forums, where our general conversational format doesn't apply, but special casing those doesn't seem like a good use of the effort (e.g, if we're going to special case something for mileage run deals, the ability to attach meta tags to the posts (especially airline, expired, cities) and then filter on those would be much, much more useful.
3. Downvotes carry danger, probably mitigated by not allowing the total to go below zero. I'm not interested in pursuing any downvote feature until we have extensive experience with an upvote only system.
That mitigation helps if upvotes and downvotes are both anonymous; non-anonymous down-votes are way too likely to be read as flames.