Quoted from TigerLaw:Having it fully turned on to begin with (thumbs up and thumbs down) is the best in my view. That way, if someone really is bothered by the thumbs down ability they can look for how to turn it off. If it is never turned on to begin with new posters will never know about the feature.
Just my two cents on the matter.
I am a little late to the party here, but I agree completely with this. Thumbs down is an important feature, even if it is misused from time to time. My best example would be when I was targeted as a bad seller by another member of Pinside and the thumbs down count to show opposition to a ridiculous claim made against me really showed how far off base the claim was (his initial post reached 40 or 50 down votes in total, maybe more). Looking back on that thread now, you can still read through the many posts to find out the story, but before, it was very clear right out of the gate that it was a baseless claim. Where one or two thumbs down don't wall very loudly(if abused) tens of down votes let the reader know right away to be suspicious as the community has spoken in one voice against misdirection. It appears that we are headed back to the thumbs down system anyway, but just to add my opinion, it is helpful and I'd like to see it again.