What about Strut B, where there's a guard walking down a corridor in plain sight? What about Strut C where there is a guard walking in the dining area in plain sight? What about the Strut E rooftop where there are guards walking around in a very open area? Both of the shell cores? There's more, these are just off the top of my head, having not played through MGS 2 in a while. Also, MGS 3 feature areas which are wider, and more open than the original. I'm just saying that if you think Twin Snakes is somehow broken by the inclusion of FPS mode, then every other MGS game should also be considered broken.
Seriously? I'm not saying every guard is out of easy reach but for the most part you have to get into position and take into consideration other guard's locations. I'll compare to some of the earlier levels of MGS. I'm going from memory but I'm pretty sure these were guards routines on normal.
Strut B: Guard outside is indeed easy, probably there more for realistic patrol routes. Inside the transformer room is trickier with two levels and the transformers themselves letting you see the guard but not easily take him out.
Heliport: Walk to right, go into first person, cap every mofo.
Struct C: You still have to get into the room to tranq him, requires very basic timing but you can't just eliminate from get go.
Tanker Hanger: a) coming in from above - go into first person view and tranq every mofo bar the dude in the lower right behind the door to the suppressor (not mandatory to advance). b) coming in from lower vent - tranq 2 guards from inside vent. go out, look up tranq others (once again bar the dude in the suppressor room).
The helipad on strut E? - what? there's containers obscuring the view of them. you have to wait.
B2 of tank hanger - works in the same way as helipad of strut E- actually works with the new controls. the starting area of mgs is also like this and works for most part.
Shell core 1 - yeah it's pretty easy but you have to loop around and time it right - not hard but there is effort involved.
Nuclear Warhead Storage - snipe all enemies from safety of other side of the lowered door when crawling.
Shell 2 core? What, this is pretty strangely designed as a area for the gameplay aspect. this was place i was thinking about when i said higher walls. guards on lower levels can't just be sniped from above.
Nuclear Warhead Storage b1 - run out of elevator, open door, snipey snipey.
As for MGS 3: foliage makes sniping harder, along with not having red dotted weapons and enemies blend more in background at times. Also there is variety with traps, dogs, animals and such to mix it up. It's amazingly well designed at times with how the game compensates for the broken weapon that is the tranq gun.