StingingVelvet: It's really not. It uses Halo's weapon and shield systems, but in every other way it's more like Half-Life. It's nothing like Call of Duty which is heavily scripted and corridor-based. While DNF is linear it's open room find the path linear, not CoD linear.
It's nothing like Duke3D though, which I assume it at the heart of what you're saying.
It's not just the level linearity that feel like (I wouldn't say Halo, but at least modern shooter) it's also the fights, low number of enemies at the same time, arenas (last time I was so annoyed at arenas while playing a FPS was while playing Jericho, and yet I otherwise enjoyed Jericho).
But I think if I was to describe DNF with a single phrase it would be : "
it plays like X but; without what made X interesting/enjoyable/etc..."
It plays like most modern shooter (arenas, little number of enemies, regenerative health); but without the scripted sequences/events/enemies or general rhythm that keeps things interesting.
or
It plays like Half Life 2 (physic puzzle, slow pace), but without "ambiance building" or story development/exploration that took place during the slower, non fight, sequences.
or even
It plays like DN3D (old enemies and weapons back, interactivity, of course Duke); but without what made DN3D fun to play, lot's of action, lot's of enemies, lot's of weapons and explosion.
The issue with DNF is not that it's a modern shooter nor that it's an old school one but that it doesn't seems to know what it is and end up being neither.