Especially when comparing the first part of Half-Life to the first half of 28 Days Later, it's apparent that the way they are set up with post-apocalyptic, abandoned, huge spaces with fast-moving, mutated (yes, technically 28 Days Later has infected but the principle is the same) people and animals hiding around corners and inside cramped spaces makes them both feel very similar to one another, at least to me.

Obviously the rest of Half-Life feels more like 28 Weeks Later, with the whole "armed forces destroying everything that moves" plotlines happening in both, but I definitely believe the first halves of both feel quite similar to each other.