[elektronaut]

Em is the iii chord in C, and it’s rarely used because it doesn’t really add any movement. Harmony is all about the cycle between tension and release, and there are other chords that do a much better job at filling the roles a iii chord could have.

It’s useful for dreamy and somewhat static chord progressions though.

[lowercased]

True. Was thinking more about the G - again, another common pianoish base (in my experience, anyway).

D/E/A - again, ime - were always "guitarish" songs. GCF always seemed to be more "pianoish". Just my own observation/experience - it may not be true of everything, but seems to be true about much of the stuff I've listened to (and tried to play).

[elektronaut]

Yeah, C major (C/F/G) can be a bit awkward on guitar, the F barre chord is a painful hurdle for new players to get over.

A major (A/D/E) is probably the easiest key to play. G major is arguably the most useful, because you have access to all the important chords in open position: G, Am, C, D, Em.