Before adding a washer and dryer to your walk-in closet consider the big bite of space these appliances will take away from your wardrobe storage. A walk-in closet may seem infinitely spacious when it's empty but these are BIG machines. It always surprises people just how cramped their closet feels when the appliances are moved in. And if you're like most folks, finding enough closet space is always a challenge. Beside the space the machines use up you will also need space to handle baskets of dirty clothes and those ready for folding and ironing. How about a folding table or counter top? So you'll just take the basket out into the bedroom to fold? Sounds complicated and cluttery. Why not put the stove and dishwasher in there while you're at it, you'll save steps!
In my opinion:
Laundry rooms are work spaces
One more thing to consider in the case of a second floor laundry area is have a pan installed under your washer--one with a dedicated drain tied to your home's plumbing system.