I also liked the Legendary Warriors themselves. My personal favorites are the twins, Kouji Minamoto and Kouichi Kimura. All of them, though, were very well developed as characters. I know what each person will do in a given situation, with the exception of Kouichi, as he did not have much time to develop after his change of heart. They all had motivation to fight even after Ophanimon told them they could leave. They all aren't perfect. Tomoki (Tommy) is still a whiner, Junpei (JP) is still a bit of a coward, Takuya remains a hothead, Izumi (Zoe)can be a bit outspoken at times (although sometimes it's a GOOD thing), Kouji needs to think about others, and Kouichi needs to stop blaming himself for things.
The fact that Kouji had a twin shocked me when I first heard about it, and the fact that the twin was Duskmon really threw me for a loop. That was a great bit of writing. I loved that, because it threw a bit more drama in. Not only did we have Cherubimon destroying the world, but we had Kouji's brother trying to destroy HIM. It was quite an unforeseen plot twist, which added a bit of spice to the patented Digimon-plot. (For those who don't know, here is the patented Digimon-plot in a nutshell: Evil digimon trying to take over the digital world gets his butt handed to him. Bigger evil digimon show up, but they also get defeated, but not before affecting the return of the biggest bad guy in the two (or 6, depending on how you think about it) worlds. Biggest bad guy gets beaten, all the worlds are happy.)
All in all, I think that Digimon Frontier is quite worthy of being the fourth season, and this, coming from me, is no small praise. I thoroughly enjoyed this season and wish that it would have some sort of continuation.