Killzone could have been much better but it's brought down by sloppy controls and frequent technical problems.
Killzone takes itself very seriously and that's one of the things that I personally dislike about it. The setting and story are too boring to be taken seriously and therefore isn't fun to experience. Comparing KZ1 to shooters of this era is a bit unfair so I'll be going a little bit soft on it, so to speak. KZ is pretty standard when it comes to content. It packs a single player campaign that lasts around 10-12 hours which is pretty average for shooters these days. It also features a Multiplayer mode which is quite barebones when compared to today's robust multiplayer shooters.
Now let's talk about the campaign. First you'll be greeted by a character select screen. For now you'll be only able to choose one character but the others will be unlocked throughout the campaign. The only thing that differentiates these characters from each other are the weapons that they use. There are also a lot of cutscenes scattered throughout the campaign missions backed up by some solid voice acting. But as I said before, Killzone takes it self too seriously and personally I found these cutscenes to be quite boring. The controls are very problematic, they feel heavy and trying to aim can be annoying. This could be intentional. Lots of "realistic" military shooters like to incorporate these type of controls which feel a bit more realistic. While these type of controls work on games like battlefield, sadly they feel like crap on Killzone. Now let's talk about another important factor in an FPS game, the levels. KZ tries it's best to be a gritty realistic shooter and it definitely shows. While I would normally be against visuals like these, I have to honestly say that these type of visuals fit KZ perfectly. As I said before as a game that wants to be taken seriously, gritty visuals are a no brainier. But sadly everything looks drab and boring, level's aren't any fun to explore as everything looks the same.
Oddly enough, even with all these problems, I can't deny that KZ can be very fun at times. I don't know if it's the way that the Helghast collapse when shot or how the weapons feel but there is something very satisfying. Speaking about the weapons, they are standard fare and fun to use most of the time. Both factions (ISA and Helghast) have their own set of weapons. Most of the time I found myself sticking to one assault rifle because most of them feel similar. Level design isn't bad for the most part but in some levels I found myself lost because I couldn't tell which way I should be going. It could be better but the basic design gets the job done. The AI could use some tuning for both friendly and enemy. They both suffer from the typical AI problems, like getting stuck or being dumb in general. From time to time my squad AI would actually do some work.
This one thing annoyed me throughout the course of my playthrough, the frame rate. It's plan awful. It's choppy as hell and frame drops are very common. Now I'm not saying that Killzone is unplayable, no. It normally holds up but in moments when a lot of things are going on the frame rate just drops very low. I understand that the PS2 isn't the most advanced system of all time but they still could have done better. Also I already talked about the artistic style and how drab it looks but I haven't touched on the technical side of things. I guess it looks quite good for a PS2 game, character models look decent, same for the gun models.
What about the multiplayer. Well unfortunately the official servers were closed down long ago so Online play is cut off. Their's still offline multiplayer though. You can either play on your own with bots or with a friend. It's very barebones and it's what you expect out of a 2004 FPS. If you enjoyed the core gameplay in the singleplayer than you will most likely enjoy the multiplayer aswell. All in all Killzone is a decent at best shooter, I won't recommend this for anyone who's looking for your average FPS, there are much better offerings on the market today. But for those who are fans of the series, I'd give it a shot.