NES
Amazing thing is that you could literally play NES games one-handed (rest the lower part of your thumb on the A button, the print of your thumb on the D-PAD and your index finger on the B trigger) in either your left or right hand even! It can also be held traditionally (horizontally) too.
-D-PAD:

-PAD
-A::A* (one-handed) or a* (horizontally)
-B::B* (one-handed) or b* (horizontally)
SNES
If they simply added x & y buttons with the a & b buttons on the lower part of the remote-controller than, when used traditionally (horizontally, like the NES method mentioned above), it could work, but they didn't. Very puzzling they didn't. I mean, why even have the a & b buttons there when they only make it possible to play NES games only...something that could already be done with the upper A & B trigger buttons already on the remote-controller. They might as well remove the lower a & b buttons if that's going to be their only (limited use) or add x & y with them. If they don't change it, then players will be required to use the retro pad to play SNES games.
-D-PAD:

-PAD* or D-PAD***
-A::a* (horizontally) or A***
-B::b* (horizontally) or B***
-X::x* (not there, needs to be added) or X***
-Y::y* (not there, needs to be added) or Y***
-L::tilt* left or L***
-R::tilt* right or R***
N64
What I find interesting is that playing N64 games will be pretty simplifed with this set-up. N64 games could almost be played without the analog attachment even (just use tilting for analog control) depending on how many face buttons and/or triggers were needed.
-Analog::tilt* or Analog**
-A::A*
-B::B*
-C-Buttons:

-PAD*
-L/Z::Z1**
-R::Z2**
GCN
Totally unplayable without retro controller 'cos some games require one or more of the following which aren't standerd with the remote-controller set-up: dual analog, D-PAD + analog control, analog triggers with digital clicks and/or free use of 4 face buttons as well. I don't see how any game that uses 2 or more of these functions at once can be fitted to the remote-controller set-up unless they were reprogramed to make use of the tilting functions, but even then you lose the analog triggers with digital clicks, etc.
* = remote-controller
** = with analog attachment
*** = with retro controller