First of all,
http://collective.valve-erc.com/index.php?go=q1_or_q2 explains what engine Half-Life uses.. Not a big issue, but it still bugs me when people say it was the Quake2 engine.
Secondly, Half-Life's story is a bit more complex that "aliens show up, military goes to kill everybody, alien head honcho gets killed" Especially when you factor the mission packs in. There was the whole conspiracy angle of that the people in charge KNEW of Xen, and how the Military was requested to pull out and a team of Black Ops was sent in to destroy the facility instead of trying to take it back. But I digress...
Anywho, I just finished reading the article in PC Gamer, and I think you'll be pleased with the progess of HL2.. The game is said to be far more open ended and you (And the aliens) will be able to do stuff like destroying ANYTHING that uses a "wood" texture, and being able to pick up just about any object and use it in some fashion. Valve looks to be encouraging problem solving on a real-world level, which should help to avoid the linearity of the original.
One example given is that you could pick up a grate and use it as a shield against an auto-turret, then use the grate to smack the turret, knocking it over so it can't shoot vertically.. Other examples included an alien who knocks doors down (Which was hinted strongly at as not being a scripted event) and being able to drop an item onto a machines gears, jamming them (Again, not scripted, but a product of the physics engine)
Extremly cool stuff...
Sincerely,
Jeremy Dunn