The setting is important - it's the difference between Star Wars and Star Trek, Star trek and Babylon 5, Doctor Who and Back to the Future. - All are Sci-Fi, but all have their unique vision and philosophies . In the case of PLANESCAPE and FORGOTTEN REALMS, its like the difference between Bladerunner and Ewoks:The Battle for Endor.
To be more accurate, it's the difference between TOS(s )/TNG/VOY/ENT and DS9. Yeah, DS9 is great - I own four seasons on DVD, I like it a lot - but it's still Star Trek, and you can't escape that, and no matter how much it breaks the mould, it's still tied into being Star Trek and infected with that flavouring.
Edit: Also, shouldn't the title for this putative NWN-in-Sigil really be Days of Our Hives?