I was 20 when I first watched this version of Mulan and I've never stopped thinking it was one of the top two Disney movies I've ever seen. Now that I've watched the underwhelming remake, I can explain what makes the animated version great.
Firstly, it's funny. Eddie Murphy in his last actually funny role, IMO, but also lots of little bits and pieces from other characters that are hilarious too.
Secondly, an awesome soundtrack. Musicals can get a bit too tiring to watch if the music isn't spot on, and they nailed it with tracks like "A girl worth fighting for".
Thirdly, ACTUALLY girl power. Like, she may have had a dragon buddy, but she actually did all the hard work herself. No magic needed.
Fourth, there was a plot. With tension and feelings and other important plotty things. A much more realistic reaction to the revelation that she's actually a woman.
And last, because I was raised on Disney, I like that she gets a very traditional happy ending (caveat, I've never watched the sequel because it sounded like they ruined the HEA).
And all of this was packed into 88 minutes.
For reference, the remake was 27 minutes longer, had none of the songs (and even stole some bits from the Prince of Persia soundtrack), had no drama AT ALL, relied heavily on magic and was almost entirely lacking in humour. So basically it lacked everything that was good about the animated version.