Alright, so I might be kind of biased and not come up with much bad
Best: The characters and their interactions. There are a lot of developed characters that develop throughout the movie. They also are really funny when they all get together. I love watching the soldiers try to out-man each other! They have great interactions and it's a really great cast.
Worst: The ending, it really kind of fell apart. We should have seen the ancestor restore Mushu to his post before Shang came and cut True to Your Heart, and then it would have been much stronger, if still not particularly good. Also, sometimes Mushu gets in the way, his part should have been cut back a bit.
What would have made it better: I described it in the "Worst" section.

Best: Characters, soundtrack, and plot. Most of the characters are developed and likeable, and do what they are supposed to do well. Even though the soundtrack is mostly a bunch of scores, the scores are extremely strong, and sets the mood perfectly. The four songs are well done too. The plot is very interesting, well-paced, and also gives us a chance to develop the characters.

Worst: Mushu. I liked him when I was younger but now that I have gained a critical point of view, he is really out of place, and takes away from the movie's serious and somewhat dark amosphere. I also think Dhan Yu could have been more developed, and given more scenes. If he had this, then he would be more of a threatening villain.

What could have made it better: What could have made this movie better is if Mushu was recast and tamed, Shan-Yu was more compelling as a character, and if the ending was better. These are only minor flaws and otherwise, this film is one of Disney's best, and is close to perfect

Best: their are a ton of things that the movie did a phenomenal job in showing, it's gonna be hard to choose the best, but I have to. For me, it's the characters in general. The interactions with each other is amazing. They know how to really make it seem live and realistic just like regular people's interactions. They all fully develop throughout the movie and you get to see the development. The development keeps adding to th character's personalities to make them more live, 3D. Relatable, and realistic. It makes the audience satisfied in the end when they see the personality all coming together perfectly. The creators used perfect dialogue to match each character's personality and they also really bring the characters to life with their actions and expressions. Each and every character plays a perfect role in the movie and they have just enough characters to bring the movie to life and complete the movie, to fill in the empty holes and add a spark. The characters are layered enough for everyone to enjoy and have characters to love and hate. I can go on, the characters are perfect.

Worst: the worst aspect in Mulan for me is the unrealistic qualities and inconsistencies. First of all, Mulan is disguised as a man, and all she does is cut her hair, wear armor, and put it up. We should have seen her maybe trim her eyebrows and do some other things to herself to make her look more like a man. Also, a Chinese army usually has much more than ten people. What I mean is, the amount of people in the army keep increasing and decreasing. It never stays the same. I also hate how during the scene of the avalanche, when Chen po saved Mulan and Shang on the horse, all it took was a bow and arrow tied to a string to pull a horse. I don't care if Chen po is the world's strongest man, but the string can't survive that. It had to have broken after pulling it even a little. Of course, it was very unrealistic that just after Mulan's hair got taken out of the bun, she looked like a girl again and we begin to see her blushing. That was something unrealistic. It was also unrealistic at the imperial city scene when Shan Yu got to the top of the building and nobody out of those thousands of people even noticed a guy climbing up. Also, aren't their guards in the imperial palace? And where did Mulan, Chen po, yoa, and ling get the women's clothing and make up?

What could have made it better: well, for one thing, we could have seen some other little snip it's of Mulan trying to look like a man. Another thing is, they could have saved Mulan and Shang some other way, they could have made the army stay the same, and show a scene after Mulan says that she is going to the city showing Shan Yu maybe disguising himself as one of those Chinese dragons or something to get inside, they could have shown Mulan probably looking in the helmet and slowly making herself look like a girl again or something, or they just could have shown scenes of Mulan blushing or looking like a girl in the army and have people get suspicious and Mulan get worried. They could have shown the guards captured somewhere and also could have shown Mulan and the others find the clothing and make up.

Jayden-G- I don't think Shan Yu only being in the movie for a couple of scenes is a real problem. The conflict of the movie was never really the war, it was always somewhat separate from the story. The war was only really there to act as a catalyst for Mulan's conflict.

^ I know, but I needed to write something for the worst section :P

Best: The characters - the movie has character development all around, not just Mulan changes during the movie, so does Mushu (and isn't it great to have a totally selfish sidekick for once?) and Shang. The father/daughter relationship is outstanding, even better than the one between Triton and Ariel, because there is not really a conflict, only a loving father and a loving daughter who both want the best for each other. I really love the quiet scenes between them.
The symbolic in this movie is great too (and the quotes related to them)- for one the Magnolia/flower theme which turns up again and again, but also the "Reflection" idea which gets picked up a couple of time. Which brings me to the music. Great Songs! Outstanding Score! Sadly both don't go together that well because the styles are slightly different, but that's a nitpick.
That Shan Yu and Mulan don't really know each other could be problematic, but since the movie manages to portray Shan Yu as a danger to everyone, and uses him like a symbol for war and tyranny itself, it works out great.
And I think that the animation of Mulan's face deserve a nod. The changes a subtle, but it's really not easy to make a female character look like a male one while still looking the same, and I think they managed it perfectly. Also, the scene in which all China bows to her.

Worst: The ending...not that Shan follows Mulan home, but from the moment Mushu turns up everything falls apart. I just hate that this movie ends up with pop-music and chickens.
The fact that the number of soldiers in Shang's troop changes more or less in every scene.
And the voice actor of Mushu. For the German version, they also picked a comedian, but one with a much softer voice. One who managed to sell the jokes, but at the same time, brought real feelings in the more heartfelt scenes between Mulan and Mushu.

What could have been done to make it better: Well, obviously a different voice actor for Mushu and perhaps one or two modern jokes less. More care in the animation, not just for the soldiers, a lot of backgrounds are very bland (it looks a little bit like the studios shortened the budget along the line and the animators had to simplify some scenes because of it). And they should have ended the movie on Mulan and Shang and started the end credits with a romantic song, and then put the part with Mushu and crickey as an interruption during the end-credits (not sure if I explained that good).

Why does nobody like the ending? I love the ending! What's wrong with it?