I never thought Halo was anything special myself. From the first time I played it I thought it was pretty average, and although I admit I had fun playing through the campaign in co-op (hell a lot of fun, we played on hard and laughed the whole time) after we beat it I never saw a reason to go back, and as far as deathmatch it was utterly outdated by PC shooters from years before it so I was bored.
I guess it was a step forward to console shooters but I don't think it was anywhere as well done as say Perfect Dark which was far more complex (although graphically inferior duh), and although some would say Halo had a bigger effect on console shooters, I would say it just had more players.
If you're talking about the first Halo game...things have changed since then. They have gone from a game where a multiplayer mode was added as a novelty to a game that focuses on multiplayer and remains on the cutting edge.
I've played Halo: Reach for hundreds of hours and can confidently say it's my favorite shooter of all time 5ever.
To me, it's perfectly balanced.
It takes more damage to kill a player than in the slew of recent military shooters, so it rewards teamwork in a manner that feels very organic without forcing the point...but doesn't make the 'lone wolf' style impossible either. The gameplay mechanics make sense...four shots remove the replenishing shield and a fifth shot to the head is a kill. Hitboxes are beautiful, I seldom feel cheated. Weapons are varied, but they all have their own charms and they never feel redundant...and they are all effective enough where I'll use anything.
The forge mode is unprecedented. People have been given an incredible tool that makes them capable of inventing new maps and gametypes - including a game mode called "Grifball," a Halo sport that now has it's own league. Two teams with hammers and swords vie for a ball in the center, then try to carry it to the opposing team's goal. Players are then given the ability to share these maps with each other, and Bungie is gracious enough to sort through the millions of entries and to place the best into it's online play cycles...so playlists change every month or two.
Gametypes range from the the very basic to the ultra complicated, and to the absolutely absurd. I've played modes that focus on single shot precision and modes that focus on teamwork on the very large scale. Classic modes like Capture the Flag, Oddball (mobile Cap the Flag) and Infection (zombies)...where survivors try to last the round equipped with shotguns and pistols while players with lunging swords chase them down...and new ones like hockey or skiball. One of my recent favorites is called Invasion. Teams try to capture three pairs of points on the map one at a time, opening new weapon and vehicle loadouts and they accomplish this. The opposing team tries to keep them from holding these two points for 20 seconds. The clock decreases when the circle is stood in until it's at 7, the last 7 seconds must be held all at once.
Maps are versatile and vastly different from one another....some of my favorites are Boneyard - an ENORMOUS gutted ship/warehouse area, and the largest map every created in Halo. Spire - a map that centers around an enormous structure with dozens of entry points and several smaller satellite structures around the base. Paradiso - A map that the Bungie staff created in it's own forge mode tool. It's an island with three structures around a mountain with a cave system at the base. Warthogs buzz around the base while Banshees fly overhead, snipers rape people from random holes and tanks battle across the center.
Everything is mitigated. There is no ultimate strategy that works 100% of the time. Vehicles can be boarded and destroyed, or stolen from the enemy if they aren't protected or careful. Larger weapon spawns take time, so there's little guarantee that any one player will whore a single weapon for the whole game as teams are constantly pushed and pulled across the map. Assholes who betray teammates can be booted, and quitters are punished by being banned for fifteen minutes after a certain number of abandoned games.
There's an easy to use theater mode, something Call Of Duty didn't pick up until it's seventh entry (and even then it sucks balls) and Gears still doesn't have, where players can capture their exploits from a variety of angles/POVs and share them with other players.
They even recently brought back classic Halo - no sprint, no jetpacks, no invisibility - for the old-school diehards in it's own playlist with some classic maps.
Kills are made more satisfying by a system of medals that denote certain actions from different killstreaks to wiping out an entire team in a few seconds. Character customization is aesthetic only, but it leads to hundreds of different combinations...even more through color and emblem customization.
The game is literally jam-packed with content.
I would argue that while Perfect Dark WAS influential to shooters and is still an awesome game, Halo has done more for multiplayer shooters than Perfect Dark by far and continues to set an incredibly high bar.
TL;DR Halo is absolute stunning, I can't think of anything I'd rather play.