The list is so long. My favorite is the internal intervalometer + ETTR. Canon has always been laughed at for not having an internal intervalometer, and ML proves how lame it is to not have one. ETTR (Expose To The Right) is an auto metering mode that allows the camera to keep the histogram pushed as far to the right (better exposure) automagically by increasing shutter time and/or increasing ISO. This is essential for doing holy grail timelapse of sunset/sunrise where the exposure is constantly changing. This feature alone is worth it's weight in gold.

However, a lot of the features exposed are more video oriented. The Canon bodies were primarily photo cameras that could shoot video in a cumbersome way. ML brings features a video shooter would need without diving into the menus like audio metering. The older bodies also have hardware limitations on write speed, so people use the HDMI out to external recorders to record a larger framesize/bitrate/codec than natively possible. Also, that feed normally has the camera UI overlay which prevents clean recordings. ML allows turning that off.

There are just too many features that ML unlocks. You'd really just need to find the camera body you are interested in using on their site, and see what it does for that body. Different bodies have different features. So some effort is required on your part to know exactly what it can do for you.