Billmon recently referred to Hezbollah having found the sweet spot between being a state (Powerful, but having many targets an enemy can strike at) and a guerrilla force (Weak, but flexible and having most of their assets hidden).
This, however, is a truly stupid use of the term:
JOE GANDELMAN looks at Republican pundits deserting Bush, which does seem to be a phenomenon. Bush -- who, as I've said before, has always been politically weak, just stronger than Kerry or Gore -- is in the "sweet spot" on the war, fighting hard enough to anger the antiwar folks but not hard enough to please the prowar folks.
I can understand what Glenn "Instapundit" Reynolds is driving at here. It's often held that if the political left and right are both criticizing you, you must be doing something right. Problem is, they may both be appalled at your stupidity and incompetence, they might have different motivations for criticizing you (Rightwinger: "You failed to glorify Dear Leader enough!" Leftwinger: "Your piece fails to be useful to the voting public!"), or they might be criticizing different aspects of the same program (The warrantless NSA spying program is not catching anybody vs it's instrusive upon citizen privacy).
But as is pointed out above, the sweet spot is a point of maximum effectivenss, a point where the ball travels to where you want it to travel to. Reynolds is pointing out that Bush has reached a point of maximum ineffectivesness. This is normally referred to as the "worst of both worlds" or perhaps the "sour spot."
There's absolutely nothing "sweet" about being criticized by both sides.