I think everyone knows the basics: that it has a duck bill, a beaver tail, otter feet and lays eggs. Less well-known peculiarities include:
Its body temperature is a good five degrees C lower than most mammals. But it’s very good at keeping it constant, even in icy water.
The ‘bill’ is not duck-like at all, it’s rubbery and packed with electroreceptors to detect the electrical fields generated by the muscles of prey wrigging in the mud – a common sense in fish, but unique in mammals.
The eggs are soft and leathery, closer to reptile eggs than bird. Despite this reptilian trait, the hatchlings are fed milk, like all mammals. But the mother lacks nipples – she sweats milk from pores and the young lap it from her belly.
Young are born with teeth, but they quickly drop out. Adults possess none.
It’s venomous – common in reptiles and insects, but unique in mammals. The male has retractable ‘stingers’ on his rear ankles, and could kill a small dog.
Most mammals have two sex chromosomes, XY or XX. Platypus have ten (XYXYXYXYXY or XXXXXXXXXX).

Amazing creature.