Thoughts, with spoilers:
It was a fairly straightfoward run after some false starts in the test chamber where the game kept plunking me down (after the teleport sequences) right next to an alien slave that would kill me pretty much instantly. They really could've put an autosave after the lengthy "start teh ROTORZ" stuff, although not right before that damn vortigaunt.
The puzzles aren't as hard as I remembered. Most are brain-dead simple. With the exception of the end of the final boss fight (Nihilanth), I didn't have any trouble with things (that last part only because I have an unhealthy full ammunition load fetish). I believe that up to this playthrough I had simply gotten past some stuff with noclip and map codes...
I also noticed details that I hadn't seen or remembered hearing before (the map that only contains the teleport to the final fight), not to mention I figured out the teleport system in Lambda Core (visited each stop, snore).Close-quarters-combat in HL (on medium) is tough. You're supposed to make use of corners. However, hgrunts are quick, and fire constantly, so they will nibble away your health in no time. My biggest gripe were the homing hornets alien grunts (i.e. big vorts) fire at you. It's nice when you get one of those guns, but hateful when those hornets are burying into your skin through walls and stuff.
Weapons:
1.) Crowbar - crates and vortigaunts/headcrabs from behind only
2.) Pistol - corner sniping early in the game
3.) Colt 357 - yay, wait not enough ammo. They nerfed the ammo count drastically for HL2, which doesn't make sense but neither does the overpowering damage
4.) MP5 - grenade launching and occasionally finishing off hgrunts
5.) Shotgun - terrible at even medium range against small targets; the spread seems to make you lose most all of the power. Decent against big non-armored things (aka the Big Momma)
6.) Crossbow ("tranquilizer gun") for some reason I never used this thing, but it gives very satisfying instant headshot kills against grunts and even the invisoninjas (assassins)
7.) RPG - not as useful as I'd like it to be. Not enough ammo to be general purpose; not enough damage to feel like you're making an impact on big stuff.
8.) Gauss Gun and Egon - I'm not sure which is more efficient - probably the Gauss but it's easy to screw up and miss with it. There's not enough ammo anyways so who cares
9.) Hornetgun - I spent half my time in Xen getting recharged at health stations, and half of what's left is waiting for this stupid thing to recharge. It's nice to pick off vorts and stuff from a distance, nice to save up your regular ammunition, but damn is it a pain
10.) Hand grenade - I don't use these all that often because they are the one thing that often triggers OHSHIT PANIC RUN responses from grunts, and then you have to switch back to the shotgun and damnit I'm already being shot
11.) Satchel - great for taking out assassins or for elevator deliveries, and you actually should have enough ammo to be able to use them when you need to.
12.) Tripmine - magnetic attraction between these and vorts or hgrunts
13.) Snark - if only they didn't turn around so often. I love these things, especially when I'm on higher ground. Probably one of my favorite weapons in a game yet - and to think I used to simply toss them into open areas below me or off cliffs.
At the end of the game, I pretty much had full ammo in everything anyway, so you probably can discount my "not enough ammo" complaints on most everything in the game. Even knowing what's coming up, it feels wrong squandering ammo - or really it's more a matter of squandering health or time, since often only corner shots or explosives save you from damage.
I dislike getting stuck on geometry. I hate having snarks get stuck on geometry, too. On the other hand, you can do something to make up for it most of the time: corner sniping enemies! That was always a tactic with the game, even when playing at lower resolutions in 1998 (1600x1200 resolution or higher just makes it easier). Many enemies just stand and snore while you shoot them in an exposed limb (sometimes even their head).
Great gameplay huh? Instead of rushing them and using corners or stealth etc. (TACTICOOL) you just shoot them when they can't be aware you're there. Hell, even DOOM made enemies run towards noises.
Counter-Strike players who migrate to CSS will notice something that relates to the corner-sniping: Enemies seem to get far too small quickly. It's probably a level design issue. It's just silly to put enemies at such a distance that they turn into a few pixels and then initiate combat. HL2 seems to be designed with this shortcoming more in mind; areas are more densely packed, you get more random shots (no sniper rifle glock kills) and clutter is usually closer to you than it was in HL1. The one thing HL2 is missing from HL1, for the most part, are the pants-pissing bottomless pits and silly tightrope walks.
You can save stupid scientists often by running alongside them and shotgun blasting whatever n00bs try to kill them. A hollow victory in any case.
Xen was nicer than I had reason to expect, although the "alien grunts in barrelz" sections can fuck off and die. Nihilanth was kind of a lol fight; I managed to clear all the places he teleports (ugh the room with just floating fireball launching mini-Nihilanths) you to (even killed the garg, although not the big swimming fish in that one) and was temporarily at a loss as to what to do after that (just shoot his head until it opens up).
Finally, this type of game brings out the bad tendencies in me - the "save all teh ammo" and "go on massive health hunting" backtrack sprees. Say what you will about health or shield regens in modern games, but they keep this sort of stuff to a minimum. HL2 also deals with it by giving you less ammo and just dropping stuff in an area that you'll want to use, instead of making you save a satchel charge through a bunch of maps to use at some indeterminate point in the future.
HL2
One thing that strikes me (after playing so much TF2) is that - in the opening section (before the abortive teleportation attempt) - the facial animations are crap when they aren't smiling or looking pensive/serious. Barney sounding apprehensive? He smiles! Also, jump on the locker Dr. Kleiner is fudging with when you get into his lab for the first time and look at his eyes bounce around.
I noticed a few new things like how the courtyard you drop into out of Barney's Beatdown Bonanza in the trainyard processing station is a big cube, and the logo on Alyx's shirt.
As always, I notice the inexorable force of noclip planes pushing you all over the blasted place in the game, and it doesn't really feel more natural than HL1's style, where characters just stand there and won't die even when you blast them in the head with the Egon gun. It makes for less aggravating gameplay, though.
One thing I want to do again is get to Nova Prospekt; I've played the Ravenholm and before areas to death, need something new.
Also, recently I replayed both Episodes (got all the achievements on Ep2, gogo env_setname gnome), as posted in twiddle's eww FPS thread. Won't have to go through them for a loooong time to come.
I have to give it to HL for the epic vistas (such as they are) and setpiece areas, which fare pretty well given the change in time. There's one area in particular (I guess near the beginning of On a Rail) where you cross an interior area to get to some power generators. This one room has a bunker set into the floor, some sandbags in the middle of the room (with a dip in the middle for jumping over), stairs past that, and on both sides there are little overlooks that soldiers can run onto to get you. As you cross this thing enemies first come from the far side, then from one of the overlooks, and finally from the other direction. Backtracking, yes, but sort of fun all the same.
