Poison: nice idea but doesn't seem to give a huge advantage in practice against monsters high enough in level to be worthy prey. It's not like the Assassin gets super-fast healing from exploration. I suppose if APHEELSIK literally halted regeneration, then when paired with CYDSTEPP or BURNDAYRAZ it would lead to cheese.
First Strike if you explore around: very handy, but just feels like playing a less extreme version of the Rogue.
Swift Hands: Great, now I can butcher everything below me in level for no HP/mana/bad status... except I can do that to a large extent anyway with melee-oriented characters (or at least well enough that the few monsters who can't be slaughtered don't really bother me). Bosses are usually level 10 anyway. I suppose it is interesting though if you make your Assassin magic-focused to the point of gimping his melee damage, or if Dracul can somehow turn your level up to 11

So, how do you play an Assassin and make him awesome?