What behaviors of your fish, shrimp, or snails did you think you wouldn't like but actually adore? Or, what behaviors did you overtime start to love, even though you disliked it at first?
Mine has to be when any of my fish -- especially my otocinclus and pygmy cories, the secretive guys -- weave through plants. Seeing them interact with their environment is genuinely amazing in a way that I cannot easily describe.
Bonus answer: my mystery snail, Leroy, has a fondness for redecorating my epiphytic plants... since I didn't use glue or string, he would somehow always manage to get my anubias out from wherever I wedged it. At first it made me so mad, but then once I saw him nudging the rhizome out of place with his mouth, I was just impressed by his dedication! But I didn't want him to end up killing my plants by distrupting them so much, so then I moved all my anubias and java fern to my 5 gallon so he will stop harassing them! (For some reason, he doesn't uproot stem plant cuttings... he just hates epiphytes!)