I don't know if there really is or not. I'm just guessing at why SS2 has remained fresher longer than DX.
Each time I play a game of SS2, I think I'm going to be bored, but then I get sucked right in. The last couple times I played DX I couldn't make it past the first visit to Hell's Kitchen.
Maybe its due to the huge amount of dialogue in DX that I've already heard a thousand times? SS2 doesn't have that and is more of a viseral experience, whereas Deus Ex is more of an intellectual experience?
I don't know really.
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I'm just replaying deus ex now, and I have just about finished Hell's Kitchen (haven't play for over a week); it's a HUGE level: the last time I played it I did it completely differently, and this time I am finding it much deeper (last time I just tried to get through this level because) the next levels are much, much better, and tend to get better incrementally.