Big Bang patch glitches, surprises, features, comments...

Unless it's doing insanely high amounts of damage, pretty sure this is intentional. All bosses should be poisonable now.

Yes, this is the case. Also, the DoT that was based on percent of monsters HP is no longer, so you can't really do what was done to Bigfoot. The thieves Venom skill has been majorly nerfed too, but now offer quite decent extra DPS on bosses.