interesting parts are figuring out how to beat people and playstyles
boring parts are gimmicky half-developed little stage specific tricks
i would counter with, there are multiple playstyles on different stages; therefore, there are many more interesting parts available the more stages we have at our disposal
also, the only reason some things are "half developed" is because of the generally unplayed nature of the alternate stages.
dreamland has stage specific tricks too - only one that allows for a special kind of DI, for instance.
however, these tricks are fully developed (or at least moreso than the other stages) - like dk's platform game, for instance; that wouldn't work on peach's, obviously, but falcon's lower-level moving platform to middle lower platform game is just as cool. so are drop cancels. so are rotating platform things on congo. and the lack of a solid ground opens up new opportunities for recovery with some characters.
dreamland: ledge DI. unique ledge di. solid ground. grabbable ledge. options are limited to ledgehop aerial or getup.
peach's: impractical ledge di because of lack of grabbable ledge. solid ground. options are recover high and get edgeguarded or recover low and get edgeguarded. utilize bumper and wedges to best of ability.
congo: no ledge di. grabbable ledge. no solid ground. options include dreamland's, but also include ledgehopping from below back to the main platform.
dreamland got gimmicks too, but because of the commonality of the stage, they've been accepted as normal things, aspects of the game that are to be expected. if the alternate stages saw as much time as dreamland, i think the same would happen for them.