[SengalBoy]

For one, they changed the formula. Usually in Tokusatsu the episodes are episodic, the heroes face the MoTW and that's it. But in Gaim, while there's still MoTW, the primary focus was the story and the lore. Each episode is important as you need to watch the next episode in order to see what happens but you also need to watch the previous episodes in order to fully understand what's going on.

Also, the series in one of the few where the writing manages to make the supporting characters, well, supportive. For example, the other Riders aside from the main ones. And also, the writing can be surprising, take a look at Ryugen. He looks like a typical sidekick but they managed to turn him into possibly the best character of the show.

TL;DR, Gaim changed a lot of things and makes the series fresh.