I'm not much one for achievements but I guess these are some possible ones:
-- Knock a Hunter from a team mate.
-- Free a team mate from the Smoker.
-- Revive a team mate.
-- Pass a scenario without a single team member going into 'bleed out' or dying.
Another thing that was discussed on another forum I frequent was the idea of having a kill counter that is updated frequently which keeps going up and have some sort of prize for the person who kills the ~6.5 billionth (I say that number because that's roughly Earth's human population) infected. Maybe a gift pass for L4D or some special merchandise.