HL2 was an FPS, comparing it to a puzzle game like Portal is like saying a hammer is better than a cloud.
Half-life is half-life, there is no reason why they would change a working formula. Thats why they make other games when they get other ideas, rather than try and squeeze new gameplay into the old games. And Im one of those people who dont get why HL2 has such a good rep. It was a good game, not special or great in any way. Left 4 Dead and it's sequel are the best FPS shooters they've made, IMO.