Halo may be innovative and have great action, but it's also one of the most overrated games...ever.
However, these people have a tendency to ignore many glaring faults with Halo.
The foundation for Halo is excellent. The controls are extremely wieldy and fun to use. The aforementioned integration of vehicles wasn't completely new, but it was done at the time in probably the best manner. The controls for the vehicles were mostly very good, if a couple were a little odd, but all wieldy. It's got fairly advanced AI as well.
So why does Halo not live up to the hype? Well it seems like Bungie, after working out all the logistics for making certainly one of the very best console shooters ever, if not the best one, just decided to rest on its laurels. Halo's level design is one of the most famous faults of Halo, and for good reason. The entire thing is wholly redundant, with level after level of extremely boring indoor areas, boosted up by moments of euphoria as you have great, open-ended fights outside with vehicles, and then Bungie sinks you down again. It's frustrating, and almost forces you to stop playing. The textures are very samey. Each section of these levels is pretty much cut-and-paste exactly the same, and it gets very boring.
That's not the only problem with Halo, though. There are two races of enemies in Halo. The Covenant enemies are an alien race. They're fairly intelligent, and will bob, duck, and weave around as you shoot at them - especially Elites, probably the most fun unit in the game to fight. Grenades are tremendous fun, and will blow up fairly spectacularly whenever you toss them into crowds. Overall, the Covenant are fun to play against, but they could use a lot more variety - I got tired of seeing the same units.
The second race is the Flood, which is made up of mindless drones of what look vaguely like pulsing brains on legs that will take over the bodies of dead humans. These tiny little flood can't really do much more than annoy you and take down your rechargeable shield a little, but the humans they take over and control are formidable opponents indeed. Unfortunately, they just are flat-out not fun to play against. They run incredibly fast, come in absolutely massive waves, are incredibly dull to look at, and compared against the battles against the Covenant, are downright boring. Some of the Flood use guns, but the majority will run at you and try to hit you. The Flood are not mentioned often as a weakness of the game, but indeed, they are one of the game's greatest weaknesses.
Also, there could be more guns. You have some of the standard fare, and then you've got some Covenant guns that you will pick up from dead Covenant, but ultimately there aren't all that many, and you will often not get to use the Covenant guns for signficant stretches of time.
Okay, moving on from gameplay. The graphics in Halo were absolutely outstanding for their time. Now being four-odd years old, they have obviously aged considerably. In any case, they don't hurt your eyes, so they're not too bad.
The sound is probably the best thing about Halo. The whole game is infused with an absolutely terrific and memorable score, and each gun sounds great and powerful. People mention Halo frequently as having some of the best music of all time, and it's pretty hard to argue that point when you listen to it.
The single-player game of Halo is a pretty lengthy piece of work, but as it's often extremely repetitive, Halo's value does get a knock. The multiplayer in Halo is fairly fun if you get four together, but it's awesome with 8+ people, when you'll have huge vehicle fights. Unfortunately, to do this you'll not only have to have multiple XBOXes and controllers but multiple TVs, plus the system link. It's not impossible, just extremely unlikely. As XBOX Live didn't exist at Halo's launch, obviously there's no support for it.
This is partly offset with co-op, which is pretty much a blast in any half-decent game, and this is no exception for Halo, especially with the vehicles. It certainly makes you wish that more developers took the small amount of time it takes to put co-op in, considering it's so easily implemented a device.
Overall, Halo could've deserved most of its reputation. But it doesn't. It stands as a fun and innovative action game for its time, but it never will be what it's going to remembered for.