raocow's talkhaus

The SMB3 aesthetic run last month made me think about level themes in Mario games so I compiled a comparison of the world themes from some of the 2D games; Super Mario Bros 2 and 3, Super Mario World, Yoshi's Island, and the New Super Mario Bros games. For New Super Mario Bros 2, special worlds are listed after the world they are parallel to. So World Mushroom is listed as the third world because it is accessed from the first and sits parallel to world 2. The actual world 3 is world 4 in the list.
As for observations, they really dropped the ball with the NSMB sub line reusing the same themes over and over, and someone at Nintendo really wants to make sure that you go to the desert second.

How about Super Mario 3D World?

mario 3d world doesn't really have world themes despite what the overworld would have you believe

It's also not a 2D game, so there's that. Non-mainline stuff.

Super Mario Land - Desert, Seaside, Mountain, Assortment

Super Mario Land 2 - Forest, Water, Toy, Giant, Haunted, Space

Super Mario Land 2 had some pretty unique world themes, didn't it? And they made a lot of one-off graphics for individual levels, too - the Mario Zone levels immediately come to mind. Also, a lot of new enemies were made up for this game that didn't show up in future games. Now that I really think about it, SML2 stands out as a pretty weird Mario game, all things considered.

The 3D Mario games seem to do a better job of world theme diversity, but it's not comparable to the mainline 2D games: they're generally more nonlinear, the worlds are more free roaming rather than consisting of separate levels, etc. Super Mario Sunshine is probably my favorite in terms of world themes. Each region of Isle Delfino had its own environment and distinct character, but they were all tied together by an overarching tropical theme.

As far as the NSMB subseries is concerned, it just seems to be more rooted in tradition compared to the other Mario games. To attempt to give a more satisfactory explanation, I would guess that the Plains are World 1 due to having predominantly flat terrain, in order to gently introduce enemies and basic game mechanics. The Desert is World 2 because the terrain is still relatively non-complex like the plains, but gives an opportunity to introduce a whole new set of enemies because of the different setting. World 3 tends to be dominated by water levels, to get the player out of their comfort zone and have them contend with altered physics more often. And so on for the rest of the worlds. This is all just speculation, because I haven't played all of the NSMB games yet. I'm sure someone else on the internet has rationalized the sameness of NSMB world themes better than I have.

NSMB's issue is that it not only recycles world themes, but from Wii on, they've literally recycled most of the music, too. Probably why I like NSMB DS and Wii more than the 3DS and Wii U ones.

Everyone sing along!