From the beginning Killzone never really seemed inspired, but neither did its only competitor at the time - Halo - to be fair. Both had their fair share of sci-fi themes (Killzone's of course being Steampunk nazis, oh how sick I am of them) that had been done before , and both are first person shooters, but that is where the similarities abruptly end.
Killzone 1 is without a doubt one of the worst games I have ever played. Some people call it a functional shooter, but it isn't. The enemy AI will literally stand in front of you and stare at you, and never shoot you even if you are 5 feet away. This doesn't happen all the time but the fact that it happens at all is a joke. You could try to say "Well the game is old." It is old, but, plenty of shooters came out before it, and alongside it, that were significantly less broken. Halo had some of the same issues if the enemy lost line of sight, but again, everything else worked fine in Halo.
Beyond that the gameplay itself honestly made me want to cry. Segment after segment after segment of mindless slaughtering Helghast, with no presence whatsoever of the overall story in the levels themselves. You got story in little bits, in cutscenes, and then it was back to mindlessly killing Helghast.
I won't call the game hard, but it's certainly ridiculous. It should not take 180 bullets from any gun (And no, I do not spray wildly, I am talking about literally hitting an enemy - at the end of the game - at pointblank range with the Helghast machine gun over 100 times), to kill any enemy - not even the heavies in Killzone 2. On top of that, you yourself take ridiculous amounts of damage per hit and then you can only replenish so much health - you have to get a medkit for more. Meanwhile, the enemies can survive 4 headshots and they just keep on coming back. While that obviously increases the difficulty of the game due to sheer broken gameplay systems, it makes any game significantly more frustrating than difficult.
So I thought it was downright hysterical at the end where Luger and Rico, in a cutscene, are one-shotting everything. That kind of shit in games just slays me. Talk about disjointed narratives and utterly breaking the suspension of disbelief.
One slightly redeeming feature is the variety of weapons and seemingly innovative features compared to shooters up until that point in time. Grenades give you a meter that tells you how "Cooked" they are, before you throw them - something you're allowed to get away with in sci-fi that frankly should be in more games. Sure, it's easy mode, but come on. The weapons are all pretty interesting, design wise, but the novelty soon fades once you realize how poorly programmed those guns all are. Add to that the fact that guns you want to use, you never get ammo for, ever. So you are basically forced into using a Helghast assault rifle in order to continue having ammo.
Throwing hand grenades becomes a chore, as you don't throw with any reasonable amount of force. All lobbed objects seem to follow the same lack of physics - grenade launchers do not arc in a remotely expected fashion. Even if the target isn't far away, the amount of curve you have to put on a grenade launcher shot is downright silly. Grenades aren't the only thing that follow this incomprehensible rule set, either. The bullets from every gun will spray out in random directions on automatic, sometimes even on burst fire- sometimes it just seems like the game decides every fourth bullet should go in a completely random direction.
The environments all seem to lack any sense of personality and frankly the concept art is more impressive than the individual levels. Obviously this is due largely in part to it being a PS2 game, but anyone who owns a PS2 knows the technical limitations don't stop the games from looking good. It doesn't feel, trekking through these environments as you lay waste to endless enemies, as if there is any subtext or story to be told by them. Contrast that with so many other games where the environments themselves - the geometry and the textures themselves seep with context such as Dead Space, BioShock, Half-Life, Deus Ex, Thief, Dishonored, System Shock 2 and so many others. It's not as big of a factor in third person games, but in first person games the environment is one of the most supremely important things to get right. It doesn't matter how many people you are slaughtering with a chainsaw.
More-so than anything else, even for a shooter, the gameplay is simply sorrowful. I've noticed this about Sony exclusives and it seems to be a trend, though most aren't nearly this simplistic. You proceed through the game almost entirely by just shooting wave, after wave, after wave, after wave of Helghast soldiers. Only near the end of the game do you start pressing buttons to activate computers and at least having the illusion that you are doing something other than pressing R1.
All of it culminates in you confronting the villain, who is hardly present more than 18 seconds the entire game, on an orbital defense platform. He escapes and literally an entire division of Helghast soldiers, most of whom are extremely overpowered, attack you. While usually you have your three team-mates, useless as they may be, you are alone for this sequence until you literally kill the entire first group of them yourself - at which point your teammates will bust in in time to help with the rest. Assuming, of course, that the game doesn't glitch out and you get stuck killing all the enemies alone, because that happened to me more than once. There is no cover, 2 health packs, no ammo, and god forbid you not have your brightness turned up. To make things better, sometimes this last sequence will completely glitch out and you can't win the game. So you have to retry from last checkpoint and put up with this utter nonsense all over again.
As far as HD goes, it really looks no different to me than the original game, other than the fact that it fits on widescreen without horrid results. The cutscenes are all incredibly blurry and look about equal to the gameplay, so I'm really not certain what they did in fact up-scale other than the overall resolution.
Sorry prospective players, this game is miserably bad. I have to be objective and give it some points because the game does turn on if you open it, and the guns do reload, and enemies do eventually die. It doesn't brick your system at least. If you want an old sci-fi shooter with bad graphics, you'd honestly be better off with Doom 3 or Half-Life or anything else. Hell, Half-Life 2 came out 2 weeks after this game and utterly blows it out of the water.