I've been playing for the first time a while back.
It's neat, but I think the primary reason people love it so much is that it's the first popular console FPS that didn't control horribly and split screen co-op. On it's own it's just a decent shooter that no longer feels special.
It was revolutionary and changed a lot in game design on a significant number of fronts. It just aged poorly as others iterated and replaced its aspects. Halo has aged well mainly because (like most Bungie titles) it still feels like a modern title due to how tight, responsive, smooth, and intuitive the player’s interaction with the world is — mainly the controls and weapons.