The ability to sprint sideways, go into bullettime, jump, pull off 2 headshots mid air, spin around, switch to crowbar, and bludgeon the bloke behind me in the face immediately upon landing.
... at least that's how Half Life 2 was for me.
A really good story and characterisation will probably get me interested, even if the gameplay is lacking. Or really excellent gameplay. Or the promise of cake.
Which Valve hates so fucking much, they never ever ever make even a passing remark to it in Portal 2. Internet... good job.