There's a lot of things about a musical that make me fall in love with it.
I'd probably have to say the number one thing is the music MUST hit a chord in me somewhere. I say this because I have been a fan of musical theater since I was about three or four years old, a good 21 years before I saw my first professional stage musical (Sorry, I'm not counting countless school musicals, or even ballets which number in the thousands.)
For so long, I only had the music itself, not the visuals to go with it, to make a musical work for me. Case in point, Phantom. I always knew I would LOVE it when I got to see it for the first time, because the music - even on the OLCR - moved me to the point where I would listen over and over and over again. Sound of Music did the same thing for me, as did a few others in those early years.
Now that I've seen Phantom numurous times, and have many, many recordings of it . . . cast makes or breaks it. Out of eight times, the only time I really didn't totally fall into it was the very last time. Partly because of a minor lack . . . a brand new phantom who wasn't comfortable in his shoes yet coupled by a Christine who . . . um, was having a very OFF night and my favorite Raoul being out that day . . . every other time I've left the theatre with a feeling of euphoria. Although I think part of the letdown was knowing it was the last time I'd see it for a LONG time.
I've fallen in love with other shows because of staging and dancing (DOTV, Sound Of Music, Aida, etc) although the music, there's got to be at least ONE song that touches me, maybe even makes me a bit misty, if not downright makes me cry.
(Which I know when I finally DO Get to see Wicked, I'll be bawling during "For Good" . . . I already do from the cast recording!)
So all in all, it's about the MUSIC and the way it makes you feel! I think maybe my screen name says it all

*I'm Inside The Music*