The Soul series wasn't the first fighting game to feature weapons, of course. In the early days of the genre, Samurai Shodown distinguished itself with its detailed sprites, gore, and katanas. Battle Arena Toshinden, meanwhile, distinguished itself as the first 3D weapons fighter, close to a year before the original Soul Edge hit the arcades.
Soul Edge, for its part, was Namco's way of testing uncharted waters. It was positioned as an alternative to Tekken and Virtua Fighter, which dominated 3D fighting at that time, and its weapons were a big part of its appeal. Apart from feeling smoother than the stiff Tekken, the strategic concept of weapon range was more intuitive than the combo-heavy 3D fighters that had preceded it.
The reason it's so easy to understand is that the concept is built into the visual vocabulary of the game. When you look at Siegfried, you see a large knight with a gigantic blade. It's instantly apparent that he has longer reach than, say, the lightweight ninja Taki. And a good player knows how to put that advantage to good use, or to minimize it in its turn.
Today, Soul Edge is largely forgotten outside of a small cadre of dedicated fans, but it did its part to lay down the foundation for the series by introducing the story as well as familiar characters like Mitsurugi. Soulcalibur V director Daishi Odashima certainly seems to have some affection for the original game. He even argued in favor of naming the upcoming sequel "Soul Edge 2," but was overruled.
Soul Calibur's popularity owes a lot to Namco's incredible Dreamcast port, which basically rendered the term "arcade perfect" obsolete. It was filled to the brim with Dreamcast-exclusive unlockables via its extensive mission mode, and it looked better than the original too. It was arguably the death knell for the traditional arcade scene in the U.S., which had already been in steep decline until that point. Even in Japan, Soul Calibur is barely seen in the arcades.
Fighting games too had fallen on hard times. Genre staples like Street Fighter still had their fans, but they become so complicated as to alienate mainstream fans. You couldn't just pick up Street Fighter III at a party, because the entire vocabulary was alien to newcomers. Even basic inputs like the familiar quarter-circle+punch can be tough to pull off without some practice.
By comparison, Soul Calibur was easy to understand thanks to its familiar archetypes like "knight" and "samurai," and the weapons continued to be intuitive and easy to use. A new players who struggled with the concept of a shoryuken could execute a sweep attack in relatively short order, and stumble upon more advanced techniques simply by mashing the buttons. Along with its lavishly detailed graphics, Soul Calibur's accessibility made it a huge crowd pleaser at parties.
You could argue that Soul Calibur II is when the franchise began to lose its way a bit. While still excellent in its own right, it also began to succumb to commercial gimmicks. Characters like Link were instrumental in getting Nintendo fans to buy into the series; but if you'll pardon the expression, the series lost a bit of its soul in the process.
The gimmicks continued in Soul Calibur III, but this time they affected the very core of the gameplay -- the weapons. To this point, Namco had remained fairly conservative in its weapon selection. Soul Calibur II's Raphael was a fencer, for example, while Talim wielded tiny daggers. Soul Calibur III, meanwhile, gave us the bladed hula hoop, which apart from looking silly was neither intuitive nor all that useful.
As the first version to be released without the benefit of a dry run in the arcades, Soul Calibur III is also rather well-known for its balance issues. Up until Soul Calibur III, the series had enjoyed a strong reputation with the competitive fighting community due to the technical depth afforded by mechanics like the guard impact as well as the highly balanced roster. Sophitia was a particular sticking point for high-level players, as her quarter-circle+b attack was nearly unstoppable in the original PS2 version.
Much of the luster had come off the once highly-regarded series by the time Soul Calibur IV arrived, which was only exacerbated by the bizarre non-sequitur that was the inclusion of Darth Vader, Yoda, and the Apprentice. Reaching for a Star Wars crossover only served to highlight how tired the series had become over the course of a decade. It only made it seem as if Namco didn't believe that the series could stand on its own without some kind of gimmick.
If it has any advantage over its competition, it's that it's still the most accessible of the bunch. Tekken has long since degenerated into a niche defined by impossibly large rosters and movesets, while the Street Fighter and BlazBlue communities are both fearsome and insular. To reclaim some of its past glory, Odashima and company will have to manage the difficult task of bringing back mainstream fans without alienating the fighting game establishment, which has been an issue over the past couple games.
But lest Namco Bandai forget, Soul Calibur will always enjoy a unique advantage over its competition. Fighting game fan or not, anyone can understand a katana to the face.