Lag, lag, lag, what a myth. I've played Xbox Live and I didn't witness any lag, and we were playing 2 player online against a gank of folks on Unreal. If Smash Bros. remained 2d, I have no dought it could go online easily.
I argued for 3D because with it you can fit more people into the stage streamlining the camera work. So the stage doesn't have to be a mile from one end to the other. Some were confused about what I meant about the 2D/3D camera work. There are basically two versions of 2D views (doesn't mean anything about whether anything in view is 2d or 3d); one 2D view gives you accuracy in jumping and landing cause the view is profile, from the side. The side 2d view allows one to move left, right, and up. The other 2D view is from above. It doesn't matter if the background and characters are 3d or 2d, the view from above puts a roof on the 3d space. The view from above allows you to see all your movement forward, back, left, and right. It can also allow you to see the jumping, just don't expect it to be as accurate, not cause it can't be done, it is just up to the developer. If you strongly believe that the 3rd dimension will take away more than it would add to this particular game then you are welcome to your opinion. For example, I believe platforming is too important for Mario Bros to be 3D. I just feel with the multiplayer fighting that 3D will open more doors than it closes. Plus the camera would not be set in stone since the background and characters are 3D. One could have 2D view with 3D backgrounds like Soul Calibur. I wouldn't suggest a camera behind the character like Mario 64, that is a 3D camera (it would work well online or in single player though).
I feel some of the people here are complaining about 3D today in this game, but if Nintendo made the announcement it was actually going to be in 3D tomorrow or later, then they would immediately soil their pants with excitement. I don't need Nintendo to dictate how I feel.