Yes.

very true.

That is easy to answer.

2D platformers have longer levels where the objective tends to be more "get to the end" so you can progress to the next stage. There can be more longer levels because developing 2D stages are cheaper.

3D platformers tend to have shorter levels where they have multiple objectives per stage. The objectives tend not to be "reach the end of the stage" oriented. Look at Mario 64/Sunshine/Galaxy as an example. 3D stages tend to be more expensive to develop, so they tend to re-use levels using differing objectives.

This is why Sonic games really didn't do as well as Mario did in 3D. Mario games adapted to the rising costs of 3D development. Sonic games tried to maintain the "reach the end of the stage" model in 3D and it didn't work as well.

Absolutely. But I think they still appeal to similar buyers which is good. However, it still is noticeable that 3D marios haven't reached the potential of the 2d ones. So even though they appeal to different kind of buyers, it is obviously still seen that Mario is a 2d game. But I think there are amazing 2d and 3d mario games. I just love Mario. Best platforming series available imo.

No.

2D has you getting to the end and you generally see a large part of a level.

3D has some specific objective and you may never see the whole stage because of the last point.

In the 2D ones, without using magic tricks, I have to play through just about every level. In 3D I just have to get enough stars to get to the end. I didn't even see a whole galaxy in Mario Galaxy.

^ So you actually meant "yes". I concur with that answer.

Er... yes, family guy is on. Cut me some slack.

Not really, no.

Same genre, different subgenres.

They are incredibly different gameplay experiences. 2 totally different kinds of games even if they both manage to portray the same mood sometimes.