Great post

3 things I would like to add :

A/ This not only applies to Mario but also Abe's series, Metroid, Castlevania and most other 2D -> 3D games.

B/ Main factor is not difficulty but intelligibility - 2D games give you almost all parameters into a single screen while 3D requires you to "see" that which is not on-screen thus a loss of focus, and to be honest, unless there is a sense of wonder, loss of interest.

C/ About the definition of games winning and losing are defined through some sort of quantification, which is so much harder to narrow down on 3D games as compared to 2D games. 2D games allow you to have a better grasp on timing and space, thus reducing the learning curve towards perfection and allowing a shorter path to perfect quantification --> perfect satisfaction.

And making claims based on vague childhood memories (at best) is ok?

Since you mention numbers, there are eight different genres in VGC's top 20 best selling NES games but only six in the PS3 top 20. On top of that, gaming wasn't limited to consoles back then, we had something called arcades, plus PC gaming wasn't about playing console games at a bearable framerate. Both arcades and PCs had their own dominant genres.

100% agree. Younger folks don't do their research.

@OP: My sincerest thanks for highlighting this. This crusade of depriving 2D Marios of these things is very strange. Business 101 says to give attention to the games that sell the most. Why Miyamoto feels the need to convert 2D fans into 3D fans is assinine (ass-uh-ten, ass-u-eleven) I just wish for a 2D Mario game that pushes boundaries like the NES/SNES days.

..or demographics?

2D Mario: 50-50 male - female
3D Mario: 80-20 male - female

I'd like a comparison with 3D World and/or 3D Land. They're 3D Mario games (and "proper" ones, whatever the hell that is supposed to mean), but the level design shares more similarities with the 2D entries.

that's a shallow comparison. most enemies at the first level offer neglibe threat level, so their quantity doesn't matter.

Your first sentence is a mystery to me? I do not think you proved anything with your research, but okay.

You're right in saying that the 3D games aren't as threatening on the outset, but there are more obstacles in a Mario game than the ones that kill the player. I'd like to see Nintendo further empasize raw, challening platforming in future 3D Mario titles, not overt fail states, and hopefully by extension take more advantage of 3D gameplay.

this, 2d mario is more pick up and play than 3d is. Fact is alot of people see the 3d mario and get intimditated off the bat and dont even attempt to try. 

ive been hearing this for awhile, but push boundaries...how? what are they supposed to do? Its not like they are NOT making 2d mario, hell we got two last year and DLC this year.