A good photo-hosting service should offer six things:
Space: Enough storage to keep all of your photos in one place over the years at low cost.
Quality: Your photos should be preserved in all their high-res glory, not compressed beyond recognition.
Ease of Use: You need to be able to upload photos easily, and edit them if you wish.
Ease of Access: Finding photos by date, by tags or by other means can be achieved without hassle.
Shareability: A good photo site makes sharing photos as easy as taking them, allowing you to post pictures to sites like Facebook, Twitter and others.
Printability: You or someone else who likes your photo should be able to buy a print or put together a photo book easily.