This might sound a bit fan-boy-ish (again), but I feel that's untrue

...
The gameplay isn't in your objectives... Objectives are a means to an end, telling the story, giving you something to go by...
It's what happens in between, that makes the game...
So, by your account, in the "stone age", they had
a) large, open enviroments, which allows you to approach the situation in any way you want (some ways are better than others, but nothing's forced)
b) the ability to adapt your gear to the situation at hand, including a suit which supplies you with 4 different abillities, which further enhances the amount of options at your disposal
c) an advanced physics system which allows you to use litteraly everything in sight as weapons or distractions (HL2 did it, but not to the same extent)
d) AI which actually acts in a "human" and believable way
Best Regards

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