Medication left about the house is bad. While a bathroom cabinet may seem like a good storage solution, bathroom air dampness can deteriorate or discolor medicines, vitamins, and the like.
Keep makeup in a cool, dry place out of direct sunlight. A bedroom vanity or dressing table with mirrors is great.
Some enjoy to adorn their bathroom with weird souvenirs and trinkets. These ornaments can collect dust and mildew if left static for weeks.
It makes sensible to have some bathtime toys for small kids. After the fun, remove the rubber ducks and plastic dolphins for the same reason ornaments shouldn't be in the toilet.
Leaving a towel in a bathroom for use before and after a shower seems appropriate. However, a used towel over a bath or behind a door can smell wet or fungal.
Same with flannels. Pocket-sized facecloths are convenient for a fast wash and splash, but they can get moist and moldy.
Bleach, polish, etc. should be stored in cold, dry, well-ventilated spaces. That's why they shouldn't be in a warm, humid restroom.