Don't make it too big. New entrants are little tots and carrying around a bag that's nearly the same size as them may look cute for a start, but will get very tiring for them. If a bag is too heavy or too big it can put a strain on their little bodies. Besides at primary school level your child won't be laden with huge amounts of homework that needs to be dragged from classroom to kitchen table.
Zip it! Make sure the zips are sturdy and strong. If a zipper breaks and isn't easily fixed then it can deem the entire bag useless
A waterproof bottom is a good idea. Kids put their bags down in a variety of places - including puddles - and sometimes the fabric on the bottom can be the first area to disintegrate. A good bottom will see a bag last the distance.
Pockets, pockets and more pockets. Lots of little compartments are good for separating lunches, books and wet togs or clothes.
Thomas the Tank Engine, or Dora the Explorer may be flavour of the pre-schoolers month, but be wary of investing to much money in a bag with a design or character that your child may outgrow.
Something bright and cheerful is good as it's easy for your child to spot amongst the dozens of other bags in the cloak-room as well as being easy for drivers to spot on the road.
Personalised school bags are a cute idea. There are lots of online services now like Stuck on You where you can have your child's name printed, and select a design and colour of your choice.