Pics from Kirby GC, The new Zelda, and Mario Baseball. Yeah, Mario Baseball.

[OB1]

That's a good example of where a game does something 2d really couldn't do very well, like GGA and its 3d towns... there aren't NO differences, but they really are minor. That's just a camera trick and it has quite limited applications since it can't be used during the game...

Kirby 64 uses all three of its dimensions. The math is still 3D. Just because you don't move along the z axis doesn't make it 2D. There are still three visible dimensions in the game.

Of course, if you're talking about the style it is done in, but what does that have to do with my point? I'm not talking about that, I'm talking about the medium it's drawn on... in this case, polygons or sprites (or sprites and polygons).

You said that the difference is in the art, which is false. Most of the art that goes into 3D games is actually 2D art, since you're making textures for every surface.

Exactly. Oh, it's also 2d because there is no height element to the game... that is, you don't actually have to aim up or down at things to hit them, only left or right...

The way you differentiate between 2D games and 3D games is by looking at the math. Any 2D game can kinda look 3D (like Doom), but Doom's engine does 2D math while Mario 64's does 3D math, which tells the graphics hardware to do 2D or 3D things, respectively.

It's just a curving 2d plane... the polygons let them do fancy graphical tricks but don't actually effect the gameplay. In Kirby 64 it doesn't even have the impact of Goemon's Great Adventure, I'd say, for how much the polyagonal graphics affect the game...

It is not just curving a 2D plane, Kirby 64's engine is fully 3D. My friend is actually making a 3D engine for our next side scrolling platformer, and even though it will look completely 2D, the engine is actually doing 3D math. Kirby 64 actually looks just like a regular 3D polygonal game, it's just that you don't move on the z axis. Four Swords Adventure, for instance, uses a 3D engine but looks 2D. So which one is 2.5D? The one that looks like a 3D game but plays like a 2D one. Pandemonium, GGA, Kirby 64, etc. It's not the best or most accurate descriptor but that's how it always is.