I only played the 2nd and 3rd games in the series so have no nostalgia for this particular game. It’s very dated now when looking at certain aspects like lip sync and animations etc, but still holds up gameplay wise.
You play a lot of the game in night vision so mainly look at greyscale visuals which helps it hold up. After a few missions, you get the thermal visor which is required in some sections but I used it sparingly.
The lighting model was impressive back in the day and it still works reasonably well. The problem is the inconsistencies, like sometimes you are standing near small lights but the guards won't see you. Some areas are dark but your light meter shows that some parts of this dark patch are in fact - lighter. So you can’t really use your own eyesight and judgment in some places, but if you keep your eye on the meter as you move, then you are fine.
Another technically impressive aspect is the cloth physics. Sometimes there’s objects like curtains that you can hide behind but bear in mind the movement and sound can also alert the guards.
You need to use stealth, taking into account both light and sound. When using night vision for clarity, you can be blinded by the lights. You can shoot out many lights to help you stay out of sight and reduce that glare, however the sound of the gun or the smash can at least startle guards. Simply using light switches makes guards suspicious, and they will go to turn them back on. This can be used to your advantage anyway.
Moving whilst standing makes more noise than crouching, but even crouch walking makes too much noise for many guards. You can control the speed using the mouse scroll so can set it slower to reduce the noise. You can also repeatedly tap which moves silently and looks ridiculous as you slide across the ground. There’s also different surfaces so wooden floorboards will be noisier than carpeted floors.
You can jump on the spot as an easy distraction. In other areas there are objects like cans and bottles that you can throw as a distraction. However it's hard to judge the effect because they often investigate where you threw it from rather than where it landed.
You can place you back to the wall which helps stay out of sight, can peer around corners, and is sometimes necessary to squeeze through tight gaps but that’s all used quite sparingly. You can climb ladders, pipes, zip lines, and wall jump once to navigate some corner walls. If there’s a tight corridor, you can stay high using a “split jump” with his legs either side propping up.
Some doors are unlocked, some require lockpick, and many others require key codes, or more rarely retina scans. You can often find the key codes from hacking computers or from guard’s data sticks acquired after knocking out/killing them. Retina scans require grabbing the guard from behind and forcefully pushing his face to the scanner.
In the first half of the game, it seemed possible to walk past some objectives so then get stuck. I followed a corridor, went through a couple of rooms and there was a computer you can hack but it wasn’t available because I hadn’t taken a detour into a side room to find the missing people. Then in another mission, I ran past the shelf with the rifle.
In the first couple of missions, there will be notes that explain locations, and you seem to need to understand them to find where you need to go. Like there was a courtyard and I couldn’t find my way out, but I didn’t realise I had reached the plaza where a secret passage is behind a bush which was explained in the notes. There is a map, but it just has some numbers for each room and doesn’t even tell you where you are; so I thought it was useless for navigation.
It’s possible to play most of the game with minimal kills. Some moments allow gun play but usually trigger alarms which can auto-fail some missions. Enemies can be defeated in one punch if you are behind and undetected, otherwise they take a few punches to knock down. Guards will take you down quickly with gunfire. In many missions all Guards seem to be alerted when a single one is, despite not raising an alarm or others hearing you. Even though Sam is a trained soldier/spy, his accuracy with the pistol is questionable. The rifle seems to be more accurate which is available later in the game and has a sniper scope. You also have grenades in some missions, and smoke grenades to stun.
At certain checkpoints, any bodies that weren’t hidden will trigger an alert. I’m not sure what effect it had exactly but it seems to imply the guards will be more alert and more plentiful.
Some missions require the use of gadgets to listen remotely. There’s some optional-use spy gadgets such as the diversion camera which can make noise and release smoke, or the optimal cable to look at the other side of doors before you open them.
It has its flaws, and there’s a fair bit of frustration with the vague objectives during the initial missions. It’s partial trial and error in other places which could frustrate some players. However, the linearity plays out like a puzzle. It's pretty iconic, holds up surprisingly well, and can be tense.