Haven't dug through the owner's manual in full yet, but what exactly are the "ABCD" buttons for?

On a related note, are there any remote shortcuts to change the image resolution, or do I have to stop and enter the setup menu every time? It's annoying when I have to swap between anamorphic and non-anamorphic bonus features.

The ABCD buttons are for whatever the software uses them for.

For example, on the I Now Pronounce You Chuck and Larry disc, there is an interactive "Friendship Test" that pops up questions during the movie and at the end of the movie it scores how good of a friend you are.

In that case, the "A" button turns that feature on/off dynamically during the movie. Basically, it turns on/off the U-Control features on that title.

I'm not sure about other titles, but I assume it is software dependent. Perhaps Universal got smart and "A" always enables/disables U-Control but I haven't tried that.

What awmurray said.

I have a couple of discs, including "Full Metal Jacket," where the A button allows you to incrementally zoom in on the picture each time you press it. Then you can use the direction keys to pan around the zoomed image.

On some discs - Terminator 2 Theaterical Cut (French import) - the buttons shrink the size of the subtitles and move them out of the picture.

The B button is normally used for bookmarking your place on an HD-DVD.

I'm not understanding what resolution has to do with this. The extras get upscaled to whatever resolution you have selected, so why is there a need to switch something on the player for anamorphic and non-anamorphic extras, and how would changing the resolution do anything? If you're referring to non-anamorphic letterbox, that would be handled in the display (the player can't know the difference between those and fullscreen 4x3 extras anyway).