To address your bad points:
-The Armstrong fight may seem tough, but there are some tricks and ways to make it easier. First, this fight is much easier on lower difficulties not just because he'll do less damage, but because he'll have less health too. Armstrong is actually the only one whose health is affected by the difficulty chosen. Also, while he's close enough to you he won't try to throw explosives, so if you can jump on top of something like a bed, wooden board or even his head (possible while he slams the floor or by jumping off something else) and stay close enough to him, he will keep trying to punch you but will keep missing. Meanwhile you can keep hitting him until he's beaten. There's also the simple technique of hitting him once or twice, then backing away and waiting for him to try to punch you, and then moving in to hit him once or twice again and repeat. If he's about to slam the floor then jump, and if he's about to throw explosives then run to minimize the damage you take. That's probably the real way to do it.
-While the lack of leaning is disappointing to some people, you can generally step out for a moment to see around corners, and then duck back quickly without getting spotted. However the closer you are to an enemy, the less time you have to duck back before being spotted. You also have less time if the enemy is alert and looking for you. This is close to the same result as you'd get from leaning, though maybe with leaning you'd be able to peak longer. A neat trick in this game though is that if you duck back right before being spotted, the guard will think he saw something but won't be sure what, will get alert/suspicious and will probably come to investigate. Like the coin, this is useful for luring guards out of position. Once they turn around and especially if they start to head back, you can easily dispatch them with a gun or karate chop. The sequel, No One Lives Forever 2, has leaning.
-The limited inventory for gadgets is a little annoying, but you can learn to live with it. For starters, you're required to take any gadget that you absolutely need for a certain mission that you won't otherwise be able to find along the way, so feel free to fill the remaining slots with whatever you want. Remember that you can, and are actually encouraged, to replay missions after you've gotten all the gadgets. You may need to replay a mission with a gadget that you didn't have at the time in order to reach secret areas and items. Likewise if you know that certain gadgets are useless in certain missions, or that you can find them later on certain missions, then don't take them on those missions. Second, you can do without many of the gadgets:
*The robot poodle is only useful in 2 mission areas in the entire game, and there are other ways to deal with dogs. Avoid or ignore them and kill any guards in the area first, before shooting the dogs. You could shoot the dogs first, but nearby guards may be alerted to your presence due to the sound of the bullet going through the bars/fence of the dog cage. All the dogs do is bark; they are harmless without nearby guards.
*Body Remover is obviously useless if you play with fade bodies on, but aside from that it's really only useful for getting rid of bodies in a camera's or searchlight's field of vision, and only if the alarm hasn't been sounded yet. Even then, you'd have to be fast enough before it gets spotted, and without getting spotted yourself. Instead there are several other options: You may be able to wait for the guard to leave the view of the camera or searchlight before killing him, lure him away by throwing a coin, letting him see you or even shooting your gun, not kill him at all and just sneak by him, or even just kill him and let the alarm sound since it's still possible to fight your way through the level (as long as you won't fail the mission for sounding the alarm). Now you may be thinking that it could be useful for preventing guards from discovering dead bodies, but let me tell you it's usually not a big deal if a guard spots a dead body. They will usually run over to check it out, and then start looking around for you. During that time you can shoot and kill them. In a few missions where you fail if the alarm sounds however, guards will try to sound the alarm as soon as they spot a dead body. In those missions you definitely want to kill all the guards in an area before moving on.
*The Barrette is useful for quietly picking locks, but remember that you can also shoot locks off if you need to. This will cause a commotion for any nearby guards of course, but as long as you can deal with them you don't really need the Barrette. Also many locks, such as those on fences, can be jumped on top of and over without having to even pick or shoot the lock. Sometimes there is another way to go to get behind the lock as well. As for the Barrette's Poison Capsule function, that is pretty useless considering that you have guns and a karate chop. The only real bit of extra functionality it provides is being like a karate chop that is able to hit guards anywhere and after they have spotted you (since the karate chop only works on the backs on enemies' heads and who haven't spotted you). But in that case it's not a one hit kill and you might as well use a gun instead.
*Sleep/Stun/Poison Gas: The main use for Sleep and Stun Gas is on that mission where you can't kill anyone, but of course you can still complete it without them. I don't recommend the Stun Gas because it doesn't knock enemies down, meaning they'll still be in your way (and also in the way of other enemies who otherwise might have wandered into it). Also after it wears off, they will know where you are and chase you unlike with the Sleep Gas. While the Stun Gas is safer to wander into than the Sleep Gas, you won't be able to see anything for a little while. Aside from on that mission, the Poison Gas is better since it can kill groups of enemies quickly and silently, but you'll lose health quickly if you wander into it. Like the other gases it has a limited range and number of charges too, so it may not be worth having it take up an inventory slot when you have guns and explosives that can get the job done instead.
*The Camera Disabler might seem enticing, but it's not that useful in practice (maybe a little more useful on missions where you fail if the alarm goes off, but still). First, you need to be able to reach the camera, and if you need to jump to do so then you'll have to time your jump due to the lengthy animation when using this gadget. Second, you need to reach the camera without it spotting you or any dead guards in its field of vision. Third, you only get 5 of them for the whole mission and most missions with cameras have more than that. Fourth, if the alarm is going off, goes off later or you deactivate it on missions where that's possible, then this gadget becomes useless. Finally, it doesn't work on searchlights. Again even if the alarm goes off, it's still possible to fight your way through the level (as long as you won't fail the mission for sounding the alarm). The Camera Disabler is much more useful in the sequel, No One Lives Forever 2, since it's shot out of a utility gun instead, you can find more ammo for it, and because alarms will reset and be able to get set off again, bringing more enemies each time.