Eric Clapton vs. Elton John

Here are ten reasons why the guitar is a more difficult instrument to play than the piano:

1. Once you tune a piano, it's tuned, and can stay in tune for years. A guitar that's perfectly in tune at the beginning of a song can be out of tune halfway through that song. Plus, the piano is always tuned the same way. The key that plays a C note on one piano will always play a C note on any other piano. Guitars can be tuned in any number of different configurations — the open top string is usually E, but it might be D, or F, or something else; it depends on the tuning.
2. On the piano, the right hand and left hand are doing essentially the same thing. They play different notes and such, but basically the same thing. On the guitar, the left hand is fretting notes while the right hand is picking or strumming them. Fundamentally different actions.
3. A chord on a piano is usually three notes, sometimes four. On a guitar, you frequently have to play six-note chords with only four fingers (the thumb on the fret hand is almost never used).
4. On the piano, there is one set of keys in strictly ascending order. You always know whether one note is higher or lower than another based on whether it's to the left or right of the other note. On the guitar, there are essentially six different sets of notes which overlap. Is the 3rd string, 6th fret higher or lower than the fourth string, 12th fret? Answer: lower, but unless you play the guitar or have one in front of you, it's not obvious.
5. Unless you press the sustain pedal on the piano, as soon as you remove your finger from the key, the note stops. On the guitar, you can remove your hands entirely and the open strings will ring unless you deaden them.
6. If someone has never played a piano in their life, you can teach them a C major scale in about 10 seconds: Black keys are grouped in either 2 or 3. Look for the 2 black keys together, and the white key immediately to the left of that is C. Hit that key, then each white key next to it (to the right) until you get to the next C. That's it. Teach someone that, and if they find themselves at a piano a month later, they could probably repeat it. On the guitar, it would be 2nd string from the top, 4th fret, then 2nd string 6th fret, then 3rd string 3rd fret, 3rd string 4th fret, 3rd string 6th fret, 4th string 3rd fret, 4th string 5th fret, and 4th string 6th fret. Or instead of 3rd string 3rd fret, you could do 2nd string 8th fret. Or numerous other ways. I've been playing guitar for 20 years and I had to do little air-guitar fretting motions in order to figure out how write it down here. Someone who had never played a guitar before would have no chance of remembering the notes a month later. (On the other hand, playing a C-sharp major scale on the guitar is easy once you know the C major - just move everything up one fret. On the piano, I'd have to think for a minute to figure it out.)
7. If you don't play the guitar often, playing for more than a couple of minutes causes the tips of your fingers to hurt. Piano — no pain.
8. You've got grands, baby grands, uprights, and other types of pianos, and they all look different, but excluding quality differences, they play pretty much the same way. Playing an electric guitar and an acoustic guitar are very different. 6-strings and 12-strings are also very different.
9. With a piano, you play a note or you don't, though you can play it louder or softer. Same with a guitar, but you can also play the note and then bend it, or hit the note below and bend up, or hit the note and slide up or down, or hit the note above and slide down, or hit the note below and slide up. You can bend strings behind the nut in some cases, and if you have a tremolo bar (aka whammy bar) or a slide, you have even more options. Plus there are natural and artificial harmonics, which are impossible on a piano.
10. The location of a note in relation to position of the black keys tells you immediately what note it is. I haven't taken a piano lesson in over 25 years, and I can't read music anymore, but if you asked me to fine a G on a piano keyboard, I could find it right away. On a guitar, you just have to know, or remember the notes that the open strings play and figure it out from there.

Note to piano players — don't get all bent out of shape. This list was made tongue-in-cheek.