Awesome Music: For a series that has weaponized sound and musical instruments, it is to be expected to have a lot. Kagayaki, the 1º opening, takes the cake.
Broken Base: The second half of the series. The general consensus seems to be that those who liked the mold-breaking approach of the first half disliked the second, but those who preferred the franchise's more traditional style prefers the second half over the first.
Mis-blamed: Many fans will often give Toshiki Inoue the blame for the show's tone shift in the second half. Thing is, however, the changes were the result of Executive Meddling by one Shinichirô Shirakura (who was involved in producing Agito-Blade, and took over the role of producer at the same time as Inoue took over writing; Shirakura is similarly disliked by Kamen Rider fans, yet strangely, this isn't one thing he gets blamed for), and if Inoue had refused, they would've just gotten someone who would've made the changes.
The Scrappy: Kyosuke Kiriya, who was so despised that when his actor was cast in Kamen Rider Den-O, the Scrappydom passed to his new character before he had even made his debut!
Tough Act to Follow: Oddly, Hibiki is this not to it's direct predecessor, Kamen Rider Blade, but rather, Hibikiitself. This is mainly due to the Retool - the first half of the series is very different from a traditional Kamen Rider series, whereas the second half is more akin to a normal one. While the first half does have people who dislike it because of how different it is, it's seen as the stronger of the two just because of how different it is.