Let me say this to start: I can't help being biased. While Dragon Knight was the series that got me to try Kamen Rider, and Ryuki was my technical first, Decade is the show that made me love the franchise.
Not that I refuse to acknowledge it's problems, mind, so with that said let's talk about the plot. There isn't one. Or at least, there isn't an overarching story. The whole hook of the series, the grand tour of the Heisei Rider worlds, is blown through in nineteen episodes, and doesn't bother developing the one thing it has resembling a villain, Narutaki during that time. Then the Riders claim there's a whole multi-verse of worlds out there so they can spend six episodes faffing about with, in order, a world ruled by the various "Evil Version Of The Hero" Riders, Di-End's world starring the Riders and monsters from the Blade movie and Shinkenger's world (Yes, a Rider/Sentai crossover) After that they remember that Kamen Rider had a showa era and start using those worlds complete with a spiffy new version of SHOCKER, DAI-SHOCKER, as an antagonistic force, but that's the last six episodes of the series.
And I guess I should probably also mention the fact that the Rider Worlds Decade journeys through are alternate universes, but this is a very minor detail in the end. While it makes for a poor tribute, the AR Worlds are all interesting What If? scenarios to their original series' (What if BOARD wasn't destroyed? What if Fangires coexisted with humans under Kiva's rule? What if Faiz was set in a school and had likable characters?).
TL;DR Decade Has No Story.
However, the lack of a real plot isn't enough to ruin the series, for me, because of the characters. I knew from the first episode that I liked Tsukasa and Natsumi, Eijirou is the chillest old guy ever, and while Yusuke doesn't really need to be there, Kaitou makes up for it by taking trolling to artistic levels.
Decade is not a perfect series, by any stretch of the imagination. The plot's a mess, there are no villains and it fumbles at paying tribute to the previous ten years of Kamen Rider. But I found it to be a fun mess, and if you can let go of the fact that these aren't the same Riders you knew, maybe you can too.
Oh, there's one more thing I should probably mention, it's really important, you see, that you *schick* *Ka-chuck* [ATTACK RIDE: INVISIBLE]