Honestly, I have to say I consider DCD to be....a first step. Toei's been doing alot of them lately. If you're not following me, think of it like this:
*rolls in the smart-board...patented by Smart Brain.*
Back when we first saw Kabuto, it was proclaimed to be rather anime-ish, and it was a consensus that that was what Toei was shooting for since, well, anime basically prints money on both sides of the rising sun pond. But in many ways, it seemed to take itself too seriously. It was convoluted, Tendou was a bit of a jerk, and then there was the ending! But we'll not get into that. The point is, they tried for that anime feel. They got bits and pieces of it right, but overall it wasn't exactly perfect.
Then we move to Den-O. Essentially the opposite of Kabuto, Den-O wasn't particularly serious. In fact the goof and comedy factor was a strong-point. Furthermore there wasn't a whole lot of over-reaching plot to the thing. They had the laugh factor, and in that sense it was very anime. However they were lacking on the story and seriousness front. Regardless, it was another step forward.
After Den-O, Kiva came along. By this time, Toei had both serious and silly, and it appeared that they knew how much was too much. That experience was poured into Kiva before they went about crafting a story. and craft they did. Now, it wasn't particularly the BEST of stories, but it was certainly more than Den-O, and it wasn't quite as serious as Kabuto, although it had it's moments. We got something more...inter-personal, with a soap-opera feel. Lots of pretty-boys. Lots of fanfic fodder. and again, the anime feel was prevalent. It wasn't perfect, but it was getting close, and it was another step forward.
Decade, being an anniversary series, pretty much broke the cycle, and I exclude it from this progression because of it.
Finally we get to W. Or Double, whichever you prefer. W was, I think, the culmination of all those steps that came before. They took what they learned from Kabuto, Den-O, and Kiva, and finally achieved the right mix of everything in a backdrop of what was already a surprisingly good premise and story. Sure, it sounded sketchy at first. But it became a gem and one of those serieses that may hold a place in many people's fandoms akin to Black, Kuuga, Agito, etc. Maybe not the best of all time (as that will surely spark a massive debate) but it's got high rankings.
Why was it so good? Well to me, I'd say it's because Toei learned from the steps before. Either that or they just got damn lucky.
Still, sticking with the step theory, I'd say Decade is a first step. It was the first time they'd tried such a thing. Naturally they wanted to go big or go home. The entire thing had a feel of a grand affair that to my knowledge they'd never tried before. It was a first step, and therefore likely to be completely screwy and full of problems. Add to this the disappointment we all felt at not seeing our favored actors reprise their roles as riders, and we get a series that is just...not good in the eyes of most.
In alot of ways, DCD is like a DC Crisis event. Which, if you've never read them, they can get kinda....weird...and make no sense. Magic Space Whistles, whiny jerks punching reality, Batman using a gun, all that good madness. It's to be expected.
Decade was the first step. I'd be willing to bet money in another ten years, he will return, or someone like him will. and at that time...it'll be a bit better. Gotta walk before you can run. Or Ride, as the case may be.