I like that if I play as an Adept I cannot take a beating but have to rely on biotic barriers and light weapons while if you are a soldier you can fashion an heavy armor and assault rifle at the cost of mobility. It challenges you to think outside the box and force you to do things differently than you are used to...
No it doesn't it pigeon-holes you into set builds and ultimately forces you play a type of character in a certain way, it's the total opposite of thinking outside the box. I'll take the Ultima Online type of character/skills builder over a D&D one every single time. In UO (back in the day) it was quite feasible to play as a Halberd-wielding Mage with a mix of swords and spells; massive spike damage from Flamestrike and the Halberd but very little stamina, if you failed to kill your target outright in the initial attack you could quite easily be screwed.
if we get the ME3 weapon system then I know that at mission "xyz" there will be that OP assault rifle that I will be using no matter what simply because is rational to do so while if the same OP rifle is restricted to soldiers I'll have to try new weapons.
That's a flaw with the balance of that weapon not with the system itself.
Also people supporting free-choice of armors (with different stats attached to them) seems to ignore that if they plan on playing rationally, they still end up using the same armors that boost the stats relevant to the class they are playing.
So if you play as an Adept you will be still using the same armors that boost biotic powers.
That's not actually true - quite often playing as a Biotic in ME3 it's a better idea to take armour parts that boost weapon damage, depending on which powers you're focusing on. You're often only better off boosting Biotic damage if you're focusing on DoT powers... otherwise you could go with a shotgun, focus on the debuff tree on Warp and spec armour for weapon damage or reducing shield recharge times. Again, it's a spike damage build, like the Halberd Mage in UO.
There's not a lot of point in taking Biotic damage boost if your build is focused around Biotic Explosions either - you're far better off taking a lighter weapon and armour parts that reduce your cooldowns.
A build with Flare or Reave though, then yeah, you might want to take biotic damage buffs.
... and that's just for Adepts ...