Syndicate soundtrack was absolutely killer. In fact, all the sound effects were pretty damn satisfying.
The drug system is pretty crucial, mainly for the intelligence increase and it was trivial to decipher in those days, perhaps we were used to a lack of hand-holding. And as a mechanic I thought it worked very well, much better than the standard sort of cooldown you see these days. Being able to dumb the agents down when you feel safe, and having to super-micro-manage them, then boost all their abilities when things get hectic was quite brilliant.
I do actually remember path-finding being pretty terrible, exceptionally so in vehicles, to the point where you generally wouldn't bother with them unless it was required.
One of the great aspects of Syndicate for me was the atmosphere and attitude (which were lost a little in the sequels IMO), love it when the enemy agent music kicks in
I grabbed it off GoG when it dropped, but sadly my game time is pretty limited these days, will definitely crank it up in the near future and see how it's held up though.
But god damn would I play the **** out of a nice remake with modern tooling. Fscking Starbreeze FPS nonsense... *grumble* ... get off my lawn....