The point that I'm getting at is that it can have an impact if you go out of your way to give it an impact, but simply changing your suit, and then playing exactly the same, will give you no difference. In essence, what changes the game is you roleplaying that the change in the suit affects you, not the actual change in the suit.
That is exactly the point. The suit is designed to enhance whatever choices you decide to make. You can run and gun without taking advantage of it (though that becomes far more challenging as you progress), but using it to its fullest is simply more fun. There is no correct way to play Crysis. They leave it up to the player. They provide the tools and you put them to use.
Why do you equate the suit to plasmids in Bioshock? Plasmids themselves served special functions as weapons in and of themselves. The suit in Crysis is designed simply to augment and enhance capabilities you already have. When you match the suit with your play style, you simply become stronger and more efficient. It has a different impact on the game than something like plasmids, sure, but that should not diminish its importance.
You are getting far too hung up on the suit, however. It is simply meant to enhance the experience and is not the sole purpose of the game. The quality of the game stems from its fantastic scenario design. The demo was limited in its scope, but as the game progresses, you'll have entire forests to explore that spread out for miles as well as large towns with 20-30 builds instead of 2 or 3 shacks. The environments become much larger and more difficult to handle. It's the freedom to approach these situations that becomes so engaging. In the second area, you have to inflitrate a town and secure a hostage. You have a general idea of where they might be, but the town before you is quite large, surrounded by mines, and filled with enemies. How do you approach it? The fun comes from figuring that out and executing.
This wouldn't matter if the core mechanics were poor, but that is not the case. The gunplay is well designed, the AI is solid, and the scenario design is immersive and exciting.
The demo presents one of the more simplistic scenarios to the player in an effort to ease them into a new way of thinking. You CAN approach most situations like any other shooter early on, but they slowly begin to teach you that you will have much greater success by modifying your playing style and taking advantage of your full toolset. If you elect to play on a higher difficulty, you'll find that the AI becomes far more capable and the game more demanding. Use of the suit is required for success.
Besides, it can be a lot of fun when mastery is achieved. Using maximum speed to launch off a cliff, switching to strength in mid air, punching a hole in the roof of a building, grabbing the first enemy you see by the neck and launching him into his friends (causing the wall of the building to collapse along with objects in the way), and then tossing a grenade on top of them while you maximum speed out of there is great fun. Creating these mini-scenarios where you put your tools to use is a blast.
I have made quite explicit the fact that I've only played through the demo a couple of times
...and yet you've formed such a strong opinion on the game.
Does your suit upgrade, or something? Do you getto the point where, say, when you are cloaked you can walk right up behind a guy and he won't know, or when you are in defensemode you can take a rocket to the chest like it was nothing? HOW does the suit affect gameplay, in your experience.
This suggests that you have yet to come to grips with its capabilities. The cloak, for instance, can hide you from enemies even if they are looking directly at you (though if they walk into you, they'll begin to fire where they believed you to be). You can immediately sneak up behind enemies and silently dispatch them.
When using shield, it acts just as a shield in Halo, actually. It takes damage for you and quickly recharges. Without it, damage draws directly from your health bar. It's very much akin to the original shield system in Halo 1 (only you have control over whether or not that shield is active).