Perhaps under different circumstances I'd be somewhat bothered by the power gap thing, but as previously stated, it doesn't because the series clearly doesn't take itself too seriously (the identity of the "God" himself is proof enough of that
).
As far as character longevity goes, I recall tons of characters in Gantz dropping like flies with barely any development as well, so I'm not sure if that's the best comparison to make, either (though admittedly it has been a while since I've been caught up to the series.... but I doubt I can expect much change from a series that regularly featured random scantily clad chicks on most of its splash pages
). IMO, Shingeki no Kyojin is currently the best "survival"-type series that really fleshed out a majority of its main cast (even the ones who died), but maybe I'm in the minority for feeling that way....
Anyways, I wonder what crazy new survival game will come up next? Personally, I'd love to see one as crazy as the battle against the cat (my favorite one so far due to how WTF it was
).