They'll be easy enough to remember. Besides, doing a fatality should have at least -some- effort required, otherwise you don't get any satisfaction out of it.
Eh, I'd prefer one button fatalities. Anyone with thumbs can do Down Down Down Circle or something like that; it's not really a challenge and for most people probably doesn't yield a whole ton of satisfaction in and of itself. The satisfaction comes from the animation of a good fatality.
My problem with current fatalities is having to look up a new combination for every fatality for every character, especially when I'm playing with a friend who's never tried MK before or if we're using new characters. I would be happy if there were a couple universal inputs that would correspond to fatalities for each character so I'm not attempting to memorize a new code every time I want to see a new fatality, but one button fatalities seem preferable because either way, the input is more of a minor unnecessary inconvenience than a fun challenge.
Another benefit to one button fatalities would be range no longer being a problem. I remember one of the fatalities in MK vs DC required me to hit Back three times while being as close as physically possible, so I never was able to pull that one off.
Nothing is secret anymore with the internet in full swing, so making complex motions and distances is pointless now. Not to mention you'll be able to watch a full fatality montage a day or two after launch on youtube.
Street Fighter 4 is the hardest fighting game to establish your own unique style in - Daigo Umehara