Since I've seen several alcohol induced deaths in my taverns, as well as the occasional lethal "non lethal" brawl, I maintain the issue is/was real. Since I play dead civs I can't afford to lose dorfs to things like that, and have thus made sure I don't appoint any poisoners. If Toady has silently toned down the effects in later releases I can't tell, since I've turned the feature off.
While it happened I tried to identify what trait would cause dorfs to die, but failed. I used DT to freeze the dorf states so I could go back and check what stats a dead dorf had, but I failed to identify any stat that ought to be responsible, unless average is considered "incredibly low".
As far as I know, injury from alcohol abuse comes from lung damage as a result of suffocation, so the victim has to go unconscious for damage to occur. Most of the time they recover and stagger off, but poisoners have a habit of running up do a vomit covered dorf that's just managed to rise from the floor and push a new goblet of booze down the throat. Rather that spreading the serving, they tended to pick on the same target again and again.
For effective poisoning, the tavern should have more goblets allocated to it than there are poisoners, the booze stockpile should be inside the tavern, and the number of guests shouldn't outnumber the poisoners too much: I'd say 1:5 - 1:10. I've never had more than one poisoner working though (in my first attempt I had a tavern keeper and a performer, and the performer poisoned guests while the tavern keeper spent years on a prayer bender instead of working).
And finally: GhostDwemer, if you read the forums you'd see essentially the same posts repeated every so often: "There's a dead dorf in the tavern! Do I have a vampire?" and "There's a dead dorf in the tavern after a fight, but the justice doesn't register a crime!"