Thinking about the whole scope of the question, I would sum it up like this: the game went beyond providing an experience, and really created a culture. Few franchises have spawned the community and complimentary resources Halo has - and when you are participating in a community as opposed to just playing a game, your buy-in factor goes up exponentially, and creates a sense of devotion you don't get with other titles.

Maybe most of all, though, it created memorable experiences. I'm of the Halo generation, having been a junior in high school when it came out. It made a huge impression on me - I still brag about a particularly impressive headshot I made a decade ago. For me, recalling Mountain Dew-fueled LAN parties in high school when we would daisy-chain four XBoxes together and have it out brings back awesome memories - memories that I just don't have with other games.

//

Yes. It was the memories. More communal time bonding over Halo with friends and strangers alike than any other video game in history for the "Halo generation." I suspect many of the Harry Potter generation feel a somewhat similar rapport and camaraderie,

It's funny that there's a "Halo community" since most people experience Halo alone or in small groups.

Like life. :) I've almost never played Halo alone and have spent > 200+ hours through the years on Halo titles.

Wow! I feel like I need a good summary of the story line.

Halo is a multi-billion dollar science fiction video game franchise created by Bungie and now managed by 343 Industries and owned by Microsoft Studios. The series centers on an interstellar war between humanity and a theocratic alliance of aliens known as the Covenant. The Covenant are led by their religious leaders, The Prophets, and worship an ancient civilization known as the Forerunners, who perished in combat with the parasitic Flood. Many of the games center on the experiences of Master Chief John-117, a cybernetically enhanced human super-soldier, and his artificial intelligence (AI) companion, Cortana. The term "Halo" refers to the Halo rings: large, habitable structures that were created by the Forerunners to destroy the Flood. They are similar to the Orbitals in Iain M Banks' Culture novels, and author Larry Niven's Ringworld concept.[1][2]

The games in the series have been praised as being among the best first-person shooters on a video game console, and are considered the Xbox's "killer app".[3] This has led to the term "Halo killer" being used to describe console games that aspire, or are considered, to be better than Halo.[4] Fueled by the success of Halo: Combat Evolved, and immense marketing campaigns from publisher Microsoft, its sequels went on to break various sales records. Halo 3 sold more than US$170 million worth of copies in the first twenty-four hours of release, breaking the record set by Halo 2 three years prior.[5][6] Halo: Reach, in turn, broke the record set by Halo 3, selling US$200 million worth of copies on its first day of release.[7] The games have sold over 43 million copies worldwide, and all Halo merchandise has grossed over $3 billion.[8]

Strong sales of the games led to the franchise's expansion to other media; there are multiple bestselling novels, graphic novels, and other licensed products. Beyond the original trilogy, other Halo games have branched off into other video game genres, including Halo Wars, a real-time strategy game produced as developer Ensemble Studios' last project. Bungie also contributed the expansion Halo 3: ODST and a prequel Halo: Reach, their last project for the franchise. A high-definition remake of the first game entitled Halo: Combat Evolved Anniversary was released in November 2011. A new installment in a second trilogy of games, Halo 4, is scheduled for release on November 6, 2012.

The cultural impact of the Halo series has been compared by writer Brian Bendis to that of Star Wars.[9] The collective group of fans of the series is referred to as the "Halo Nation".[10][11]

http://en.wikipedia.org/wiki/Halo_(series)

The series centers on an interstellar war between humanity and a theocratic alliance of aliens known as the Covenant.

So essentially it's freedom fighters vs religious zealots?

What was the Flood?

Merchandise has sold $3 billion in addition to 43 million copies of the game being sold.

The flood was ... Hell...a parasitic-like creature that devoured and twisted everything it touched.

Anyone who has played the "Library" level remembers just how freaky the first introduction to the flood was; especially playing on "legendary" or expert level.

The halo nation and playing halo is inseparable from my childhood and both the story and the multiplayer memories will last a lifetime. :)