Look at the effects animation. Mickey pours just a drop of "bravo pronto" into his test tube, and it fizzes, bubbles, explodes, sputters, changes from one vivid colour to another - with each drop of the potion separately drawn and travelling in a convincing arc. Or look at the scenes in which the background moves and (in effect) EVERYTHING within the frame is animated - all without computers or even, in this case, rotoscoping.
Mickey plays a small part in this cartoon: he's just the brewer of a courage potion, which enables a fly to turn the tables on a spider, a mouse on a cat, the cat on Pluto, and Pluto on a dog catcher. His lack of screen time might strike the uninformed as good news. It IS, for this particular story, good news, since it means that Disney was quite content to assign his star character a supporting role without artificially expanding it - but remember that this cartoon dates from the days when Mickey was vital and energetic, rather than insipid.
The charm of this cartoons others like it is hard to explain. It's amusing at times, but certainly not laugh-out-loud hilarious; it's cute, but not particularly sentimental or deeply moving; the art direction is detailed and convincing, but shaky here and there; the story has a pleasing shape ... but it's not THAT great, is it? Yet the overall result is undeniably the equal of later cartoons that ARE hilarious, moving, exquisitely designed, brilliantly plotted. What does this have that so many other cartoons (including a number of Disney cartoons from the 1930s, although if you search you'll also find many that are just as inspired as this one) lack? Thoroughness? Sincerity? Something else I'm missing? Probably all three.