A laundry tub or some sort of work station with a sink is a huge help.
A tile floor for easy cleanup helps a ton.
I would not build the surrounding wall from standard drywall, use greenboard, mold resistant armour whatever its called (sorry its been a while since I built mine.
You mentioned outlets. Dedicated circuit for the fish room is always a plus. GFI is a must.
The drain idea, RO, etc that was previously mentioned are great ideas as well.
Anything that you can think of that you might use for the tank, add to the room. The less you need to trek through the house with on the way to the fish room, the happier the family will be!! (My wife hates "stinky" fish stuff coming through the house) A fridge for your frozen foods, etc is a great idea as well.
Oh, and lots of shelving/organization so it doesnt become a mess!!
As for problems- I find that reaching from the back of the tank to the bottom front of the tank is a pain in the butt. If I were to redo mine, I would consider finding a way to access the tank from the front as well. Granted, mine is 32" tall, so it makes it a little more of a reach.
Thats about all I can think of off the top of my head quick.