Any key will do for me, though naturally I prefer the more "guitar-friendly" keys like G C D A E and related minors and modes.

If I may make a point about capos, in addition to the above well-stated arguments, the use of a capo allows the exploition of open strings. This simplifies sustain, drones, widely voiced chords, and so on.

I have been an avid user of capos since I first started playing guitar. Some songs sound better, regardless of key, with open chords, others with closed chords. However, as easy as it is to get closed chords without a capo, it is not possible to get open chords in any key without a capo. Therefor to get the open chord timbre, feel, sustain and ease, a capo is necessary.

An aside here, as alluded to in an above posting, double capoing is possible, and not too hard on intonation if the capo is placed properly (though some tweaking is likely necessary). A partial capo used above a full or partial capo can create open chord sounding tunings while maintaining standard tuning string intervals up the fingerboard. Strumming the open strings create a chord based on the positions of the capos. I have capos which form Em, Amaj(which when inverted sounds like dadgad up a tone), drop E (which can be used anywhere, not just at the second fret), and others which I use with these to get an Am, Emaj, etc. Other capos include those which I use for altering open G tuning. If you're interested in this stuff, I'd be pleased to hear from you. I figure I've got the cost of at least another decent guitar tied up in capos, and I admit I don't use them all the time, but I sure do love tinkering with them.

Peace, Mooh.