Alternative Character Interpretation: Is Tendou a God-Mode Sue played straight, with his boundless confidence and superhuman abilities (even without using the Masked Rider System)? A Parody Sue and partial deconstruction of the type (with his various failures, his strange obsessions, his inability to truly connect with others)? A Decoy Protagonist for Kagami (because Kagami is the one who actually espouses heroic values much better than him)?
Broken Base: Pretty much the only things that people can agree on is that the fight scenes are awesome (when they have money to throw into the clock-up scenes) and that the later episodes are weaker than the earlier ones. Beyond that, you'll see arguments about every other aspect of the show, for example:
Tendo is either an interesting and badass character with plenty of awesome moments (and some good Heroic B.S.O.D. moments later on), or an arrogantGod-Mode Sue who is hard to root for (and think Kagami should have been the lead).
The show's tone: Appropriately tense and dramatic, or inconsistent and suffers from too much Mood Whiplash.
Decoy Protagonist: Some argue that Gatack and Kabuto could easily switch roles as The Hero and Second Rider, considering Kagami's more heroic and had quite a bit of character development while Tendou's, well, Tendou.
Ending Fatigue: It's generally agreed that the plot haemorrhages coherence around episode 30.
Ensemble Darkhorse: Kagami spends most of the series being a complete Butt-Monkey. But once he proves himself worthy of the Gatack Zecter, he becomes far more heroic in almost every way than Tendo, even as Tendo continues to be the series darling. The series does, however, lampshade that in some ways Kabuto and Gatack are equals and that Tendo respects Kagami, and perhaps only Kagami, because the Gatack Zecter is the only one the Hyper Sword doesn't steal, leaving it to belong only to Kagami.
Tsurugi Kamishiro is one of the more popular comic relief character in the series. His Large Ham tendencies and overall goofiness trying to "learn how the common man works" help. His storyline ending helps even more.
Gateway Series: This series has introduced many Western viewers to the Heisei Kamen Rider series with its balance of humor, drama, and action. The Clock Up feature really seems to help as well.
Hilarious in Hindsight: Hidenori Tokuyama/Yaguruma will be the lead star in a movie based on the boxing manga Thanatos: Mushikera no Kobushi, which will also star Yūki Satō/Kagami possibly as the antagonist.
Yusuke Yamamoto/Kamishiro Tsurugi will play another eccentric "prince" AKA Tamaki Suoh in the Ouran High School Host Club live-action adaptation.
Ho Yay: Subtle with Tendou and Kagami, but more blatant in Yaguruma and Kageyama, especially after they become the Hopper Riders.
Incest Yay Shipping: Bordering on Incest Subtext, Tendou and Hiyori had a lot of Ship Tease, and in one of the earlier episodes Yumiko asks Hiyori if Tendou is her boyfriend, which Tendou himself doesn't deny. Furthermore, Tendou's utter devotion to a girl he barely knows (who also happens to be his sole motivation with getting involved with ZECT and the Worms) seems a bit too intense for someone who's supposed to be his sister. Sean Wiig, the actor of Tendou's child self who obviously hasn't watched the series to know better, even thought that they were boyfriend-girlfriend in a post made on his blog.
Narm: Depending on who you ask, the Dark Kitchen arc. To explain, the writers seemed to be trying to find a way to work Tendou's cooking skills into an actual episode plot instead of just being a source of witty aphorisms, and have him face off in a cooking duel against a Worm who plans to take over the world with his own cooking skills...somehow.
Speaking of which, Dark Kabuto's blood-curdlingly child-like smile upon entering Earth in Episode 42 is enough to keep anyone up all night. The creepy music doesn't help either.
In a case of serious Paranoia Fuel, a Worm can become an almost perfect copy of anyone, including you. They are also stronger than you, can spit goo on you to kill you, and can slow down time to prevent you from escaping their clutches. The only way you can sense a worm is by their smell, which would be too late by then (unless you're a Kamen Rider, of course.)
Paranoia Fuel: The Worm. The fact that a race of over-6"-tall, hyper-advanced aliens with retractable tentacles for hands have successfully managed to infiltrate Japanese, and, by a larger extent, world society, perfectly imitate the humans they encounter right down to their memories, and kill those they replicate, can be severely unnerving. They're like the Kamen Rider equivalent to the Yeerks, only even more horrifying.
Take that Up to Eleven with Tsurugi being the Scorpioworm without even realizing it. The only thing more unnerving than someone you know being a Worm is the possibility that you are a worm yourself and you don't even realize it.
Special Effect Failure: Gattack's battle damage in 44 is very obviously just extra bits stuck to the suit, with obvious edges.
There's various instances of transformations not melding very well (actor and suit actor standing in obviously different poses), sometimes bluescreen effects are sort of obvious, and some Worm are designed and fabricated better than others. Let's just say, some episodes obviously had higher budgets than others.
Superlative Dubbing: The English language dub that aired in the Philippines isn't without its problems, but some of the dub casting does the characters remarkably well - particular highlights include Yaguruma, Kageyama, and the laughing tofu merchant.
Visual Effects of Awesome: The special effects are extremely impressive for a Tokusatsu series. From the cast-off and clock-up sequences to the choreography of the fight scenes, Kabuto, even after 11 years since it originally aired, is still considered to be one of the best-looking series in the Kamen Rider franchise.