In Becoming (part 2), what do you think Whistler means when he says, "Wrong, kid. You got one more thing [to loose]," in response to Buffy stating that she can deal with everything because she has nothing left to loose.

he means the last thing she has to lose is her life (:

I think he was talking about Angel .. Her having to kill him

I think it has two meanings:
1. That she still has her own life to lose.
2. That she'll have to give up on any chance of getting Angel back, or Whistler had foreseen she'd have to send Angel to Hell while he was ensouled.