Gosh, this whole debate is just so difficult to try to figure out.
Destiny Islands: we know there's a whole town from KH2 away from the little play island that we saw in KH1 but there seems to be not much of anything else going on there. Maybe the town was a part of Daybreak town or something?
Radiant Garden: it's been focused on as a research facility for heartless and had importance to several characters in the series, but they haven't shown anything but cutscenes regarding it. I feel like it is important enough to be playable, but what else could we do there?
Twilight Town: clearly there's a lot more going on here than in 2, between resurrecting Roxas, the two darknesses we've seen residing here, the mini games we know exist here, and it was the first world we really saw playable.