Potentially, Persona 3 could have been a train wreck. A spinoff of the Shin Megami Tensei series, it features randomly generated stages, with a huge focus on dungeon crawling. One only need to look at the Western reviews (and sales) of such conceptually similar titles like Azure Dreams (PSOne), or The Nightmare of Druaga (PS2) to see that Western JRPG fans traditionally haven't cared for these types of games.
Furthermore, it takes place in a high school, allowing the main character to interact with their classmates, join clubs, and socialize -- all elements of dating/life sims that are popular in Japan, but barely heard of in the West.
Of course, from Japan's point of view, it wasn't the first time that someone tried to combine life-sim elements with an RPG -- Sega's immensely popular (again, in Japan) Sakura Taisen series popularized the mechanics through its many instalments. One of the only similar games released in America was Atlus' PSOne RPG Thousand Arms, which tried the same thing on a more limited scale, with disappointing results.
Taken separately, neither aspect of Persona 3 would've stood on its own. The dungeon crawling is repetitive, and while the battle system draws heavily on the same strategically brilliant system found in most of the other PS2 Shin Megami Tensei titles (Nocturne, Digital Devil Saga), the player can only control a single character, drastically limiting the strategy that traditionally made the series so appealing.
The life sim part, too, is scaled down -- this style of gameplay pretty much began with Konami's Tokimeki Memorial, which offered over a dozen statistics to monitor in order to shape your avatar's personality, while Persona 3 only offers three. Yet, both portions come together so brilliantly that they add up to more than the sum of their parts. There are plenty of clubs to join, and numerous NPCs to befriend or even date.
Socializing will enhance the strength of your Personas, the mythical creatures that dwell in your mind and provide your special attacks. The life sim segment of the game is essentially a character creation system -- usually, these are reduced to impersonal menus, but these have been removed in favour of something more involving, and ultimately, rewarding.
The extremely innovative scenario also goes a long way towards giving Persona 3 its charm. As a transfer student in a new school, you and some of your fellow classmates have the ability to sense the "Dark Hour," a mysterious period of time that occurs at midnight, where the rest of the world lies asleep and unaware. During this time, a huge tower called Tartarus warps and mangles the interior of your school, which is somehow tied in with a mysterious apocalyptic prophecy.
A lot of the enjoyment comes from trying to balance your school/social life with your demon hunting life, not exactly a typical dilemma faced in most RPGs. It also provides an interesting glimpse into the fantasy life of a modern Japanese teenager, as the game is filled with stylish artwork and a J-hip-hop soundtrack that's alternatively catchy and grating.
Persona 3's big pseudo-controversy stems from the method where the characters summon their Persona -- they bring a gun to their head and pull the trigger, forcing their spirit companion out to attack. It's cool, in a punk kind of way, but the relative obscurity of the title allowed it to fly under the radar of the self-appointed culture warriors. This off-the-wall originality helped it earn rare accolades from the Western press.
It is more than a bit unorthodox, but Persona 3 has a lot to offer gamers who aren't afraid to try something that strays heavily from the beaten path.