The thing about Halo that makes it significant isn't readily obvious or apparent - it's the combat system and the AI.
Basically, Halo has the most effective sandbox AI for an action-shooting game out there. This is most apparent in the Firefight modes of Reach. Games such as Call of Duty are heavily scripted to give an appearance of intelligence on the part of enemies. In Halo, encounters can play out in a very large number of ways because it's very dynamic.
If you just play Halo for a single pass on the easy difficulty, you'll never see it. Playing on harder difficulties, and in co-op with a team of players, changes the experience. It's why in Halo 3 and Reach, co-op campaign has become its own thing, complete with a scoring system, leaderboards, and game mutators (skulls) - it's meant to be played over and over, not just a single pass to see the story then forget about it.
The multiplayer in Halo, meanwhile, is significant because of its flexibility. There's no console shooter with as many game set-up and editing options as Halo; same goes for the variety and balance of the weapons in the game.
Halo isn't trying to be realistic and that's not the point; what it is trying to do, is be a very deep experience that gives more over time if the player puts more into it.
Halo as a whole has not so much been over-rated, as overhyped, for the wrong reasons. Microsoft has tried to ride Halo into the ground because that's what they do - they're Microsoft. Therefore, they've tried to overplay and oversell Halo for being "bad assed" and for its story, and for its primary character (Master Chief and Spartans in general). Much of the distaste for Halo being overhyped originated back in the day with Halo 2 - a game that Microsoft pushed to the moon, but is the most incomplete and broken game in the entire series because MS forced Bungie to shove it out the door long before it was finished. There's half a game's worth of content missing from Halo 2 that died on the cutting room floor.
Ironically, a game like Call of Duty may service the casual player a lot better than Halo, because Call of Duty has become eponymous with a "movie like" and Michael Baytastic thrill ride through a relatively short but action-packed single player game. Its online multiplayer also sacrifices a lot of variety and depth for its attractive RPG grinding gameplay, with micro-rewards to keep people playing a little every day. I am not criticizing Call of Duty; just calling it what it is.
A lot of Halo fans (who aren't just fanboys of Mister Chef, but fans of the gameplay itself) tend to be a lot deeper into the nuts and bolts of such games, based on my own experience. It makes sense, since the last couple of versions of Halo have featured complex game type editors that allow one to create their own game variations and even customize maps to a deep degree.
Also, as far as graphics go, another reason why Halo's visuals are not as "impressive" as something like Call of Duty is again because it isn't as linear. Its battles and areas are larger, more open, must be playable from many angles of attack and under many circumstances, filled with vehicles, and employ dozens of advanced AI units in campaign and firefight modes. That takes its toll and requires a lot of tradeoffs. To make up for it, Halo does tend to have a great, meticulous attention to detail that stands up to repeated examination. The design and animation of weapons in Halo, for instance, is some of the most complex in any first person shooter - even if they don't "look real" at first glance because they're fictional and not based on real firearms.