There have been worries expressed on both sides of the political spectrum about the use of drone killings against Al Qaeda, and more especially so as it's come out that the Obama Administration has a secret "kill list" which even includes American citizens who are working with Al Qaeda overseas (as was the recently killed Anwar al-Awlaki).
It seems to be that there is a legitimate worry here. In a sense, drones are the modern American equivalent of pillars of the Victorian British Empire such as Charles "Chinese" Gordon -- gallivanting about the world to put down disturbances wherever they occur. However, they're also relative unobtrusive and cheap. Thus, I would imagine that there is more danger of them being used to embroil us in conflicts that we really don't want to be in. (Which, come to that, is more or less what Gordon managed to do for the British Empire on an occasion or two.) While I think that US hegemonic power, like that of others such as the British and Romans in the past, is generally a positive force in the world, power is often a temptation to over reaching. Putting international intervention only a joystick away, without any need for congressional approval or oversight, seems to put just a bit too much power in the hands of an already imperial presidency.
At the same time, I don't find myself all that persuaded by that slippery slope claims which many have made in regards to drone attacks. The argument often goes: If the president (or some secret committee not even overseen clearly by the president) can order the killing of US citizens by drone strike without trial, are we still a republic of laws? Are we suddenly just one step away from a semi-dictatorship in which political opponents and other undesirables are assassinated at will? Do we need to worry about when drones come for us?
It seems to me that this glosses over the fact that drones are essentially a battlefield tool, albeit one that allows us to enter battlefields without deploying soldiers. When American citizens get mixed up with forces that the US military has been deployed to fight, there's never been any hesitancy to treat them the same on the battlefield as any other enemy soldier. As such, I don't think that the use of drones against Al Qaeda puts us on a slippery slope to some future president using drones against his political enemies any more than the long held ability of the president to order air or missal strikes against specific targets puts us on a slippery slope to the present bombing his opponent's party convention.
The worries that we should have over drone strikes have to do with their making it too easy to go to involve ourselves thoughtlessly in regional conflicts, not that it puts us on a road to some sort of military reign of terror.