I believe I've heard Mr. Paul Reed Smith (in person on a couple occasions, too) say something along the lines of "It has to look good, or you won't pick it up, it has to feel good, or you won't play it, and it has to sound good, or you'll put it back." He gets good mileage out of that observation, but it's 100% true.
Similar to his oft-used "The guitar is the perfect drug because when you play it, you're in no pain, and when you put it down, there's no hangover." That said, PRSh didn't factor in the longing to have our hands on the instrument when away from it, or Gear Acquisition Syndrome!
Edit: I'd generally agree with Paul's take, though appearance is (somewhat) less of a factor than feel and tone. The feel is undoubtedly most important to me (I have to be able to play fluidly without feeling hampered by the neck), and that's some combination of neck carve, fingerboard radius, nut width, fret size, string gauge, setup, etc. Tone is a close second, as hearing the sound in my head realized in front of me inspires me.
To me, there's three things an instrument can do...
1) It can fight you and prevent you from expressing yourself musically. This is subtractive.
2) It can get out of the way and let you play with no interruption. This is neutral.
3) It can create a positive feedback loop of sorts where the feel and tone of the instrument inspire you, musically speaking. This is additive.
Number three is the goal.