So, my question is, exactly how different do the past series feel compared to the worlds I was presented in Decade? Feel free to say as much as you like without spoiling.
Theoretically, each world kept the main theme of each show, even if the story and characters were very different. In practice, that doesn't work very well, especially because some shows don't really have a consistent "theme" throughout, or when they had one, didn't focus on it nearly as much as the Decade version did.
In addition, the worlds tried to recreate the atmosphere of the original shows (although with the different cast and characters), but only some were successful in doing that (Kuuga, Agito, Kabuto).
Some worlds followed the original stories close enough to spoil some plot twists, but others had plot twists that had nothing to do with the originals... so, it doesn't really spoil anyone unless you already know the spoiler.
The lessons that the alternate Riders learned after each of their arcs were generally meant to make them closer to the original versions, like Kuuga deciding to protect everyone's smiles, Blade fighting for his friends and not for a job, etc. But some of them were just very different from the originals, even after the lesson (like Prince Wataru) and many people were annoyed by their "flaws", even if they were just there in order to allow them to go through that development.
And, of course, although alt.Kuuga was generally well received in his own arc, later episodes, often ignoring that he was a Rider or even part of his character (Nega and Diend worlds) just annoyed people and left many with a negative opinion about him.
Some of the random characters who appear from other, unseen, worlds were directly based on the previous shows though, keeping even casting often (The Hopper Riders in Kuuga World, Kaixa in Kiva, Wataru Kurenai in the first and final episodes, Kenzaki Kazuma in the final episode and his Movie Wars cameo, Agito from All Riders), although their roles had little to do with their roles in their own series.
Black/Black RX actually had their original actors and Black RX basically followed the original show (they even refused to recast Kazumi no Joe after they were unable to bring his actor back, just making him mentioned by the script).
The one big exception in the first nine worlds was Den-O. The arc actually has several differences from actual Den-O itself (like changing outfits with the Imagin possessions and the wish mechanic being completely ignored) but every character there was from Den-O itself and it tied into the first Chou Den-O movie.
While I'm at it, would I be correct in thinking that All Riders vs. DaiShocker should be thrown out the window in terms of Decade show canon?
All Riders is canon. Takes place between Amazon and Rider War World. Stated by some official material and there are a few hints in the show itself (like Tsukasa suddenly stopping complaining about not having a world in the two last episodes and Apollogeist never calling Dai-Shocker reinforcements from other worlds).
It's also referenced in Movie Wars (the reveal regarding Eijiro's double personality, Rising Ultimate Kuuga, Double and Decade recognizing each other).
DCD does away with it's on canon because of it's bad writing.
The decision to go with reimaginations had nothing to do with good or bad writing. It was announced before the show started and every writer who worked on the show clearly had to follow that directive.
