A look at any of the news media maps showing areas under ISIS control looks appalling, but the speed and extent of its initial success may also be an indicator of its vulnerability. Consider: in under a week it seized much of the north of Iraq, though it has not fully conquered every town along the way. Its predecessor, Al-Qa‘ida in the Land of the Two Rivers, also found itself overextended and also challenged within the Sunni community. With a logistical tail extending back into Syria and its forward positions outside Baghdad to both the north and west, it's a thin, long line to maintain. Now that Shi‘ite resistance is firming up and hardening, its momentum is dwindling.
And along much of its long, exposed left flank lies another potential challenge: the Kurdish peshmerga. Hammer, meet anvil.
ISIS doesn't fight like a conventional military force and conventional military considerations may not apply, but if I were ISIS, I might worry.
UPDATE: Try this map for instance. Feeling surrounded yet?