The titular rider (and by extension, Gills and Another Agito) is one of four primary Kamen Riders in the entire series (and the only Heisei Rider) whose powers are not directly related to the monsters fought in this seriesnote The other three riders being Kamen Rider X, whose powers were originally intended for marine exploration; Kamen Rider Super-1, whose powers were originally meant for space exploration, and Kamen Rider J, who was created by benevolent earth spirits - while the Lords were created by the Overlord of Darkness in response to his brother, the Overlord of Light, giving humanity the Seeds of Agito so that humans could become more powerful than his brother, the Lords themselves do not use the Seeds of Agito as the source of their power. This makes Agito an abnormaly on the Cross of Fire.
Milestone Celebration - This series was made during Kamen Rider's 30th anniversary and contain homages to past Kamen Rider Series. Some of the monsters are based on old villain's, and you can make the case the lead riders themselves are based on previous Kamen Riders. It also homages the Metal Heroes series which Kamen Rider took the place of and incorporated some concepts.
Older Than They Think: While Tackle's status as a Kamen Rider continues to be debated upon, Femme is not the first female to be shown transforming into an undeniable Kamen Rider form, and Shuki isn't the first female Rider either on TV. Both of those honors go to Yukina Sawaki, Shouichi's older sister and the first Agito, who is even shown in a flashback to (unwillingly and painfully) transform into Agito Ground Form. While she did not become an active Rider like her brother or her more well known successors did, this is still an undeniable case of the first female Kamen Rider, at least within the Heisei Era
What Could Have Been...: As noted on the main page, Agito was originally intended to be a sequel to Kamen Rider Kuuga, but the writers of the latter series objected, stating that doing so would have made Godai's battles meaningless.