Portal 2 added co-op as the big gameplay change vs. 1.
But Gabe has said Half-Life specifically is what they consider their ground breaking series now. So there won't be a 3 until they can push a boundary again like the first 2 did.
From all the leaks, it sounds like the big change for HL3 is currently advanced materials – destruction, traction, behaviour, conductivity, etc. This would make sense as it plays into the material work in Portal 2 and the deformation work in HL2:EP2-3, as well as the physics puzzles in HL3. Additionally it sounds like gravity may be a core feature of the game.
This is all speculation based on code leaks, and it's possible this is all true and they still don't release anything because the project stalls. Supposedly the game has moved into the later stages of development, but I'm not fully convinced by that part of the leaks.
But Gabe has said Half-Life specifically is what they consider their ground breaking series now. So there won't be a 3 until they can push a boundary again like the first 2 did.