You really need to take a look at this : http://www.members.shaw.ca/halflifestory/timeline.htmI realized the story was rather sub-par
Anyway, what was really interesting in the Half Life 2 story was the fact that it just went beyond Half-Life's story and generic shooter's stories, and instead included deeper themes, like dictatorship, manipulation of the masses, ecology, human experiments, etc... It showed pretty much every important aspect of a authoritarian political regime, and did it very well.
For anyone willing to take a look beyond the main dialogs, it was a very interesting story. Just listen again to the Breen speeches about instinct, it's a very interesting exemple of how good speaches can influence people's opinion.
On many aspects, the story was a good mix between War of the Worlds (Alien Invasion) and 1984 (Methods used by a Dictatorship), even if the story didn't cover the same details.
I don't know how people can compare it with Halo's story. It hooks you like nothing else, but it's just a cheap Sci-Fi story about humans fighting aliens in space, with some excellent inspirations and art assets. But in the end, it's just a really simple story about how we should kill the aliens because they're a threat and they're mean.
The religious themes behind the covenant are insteresting, but for what I've seen it doesn't go very deep and the entire Halo story is build on the false belief of a single alien leader.
And I tend to agree with the original poster. While I have played other games since Half Life 2, no other game except maybe Portal was as much fun and interesting to play. That's why Valve fans are expecting so much Half Life 2 : EP3, I guess.
(On a side note, even if EP2 was good, I tend to agree with Yahtzee on the idea that it used too much the same set pieces)
As for the vehicules : I hate the vehicule controls in Halo. I guess the way you enjoy the vehicule sections pretty much comes down to what you're used to. But if you know how to drive vehicules in Half Life 2, I think that their controls are more precise.