Making it a spell makes it a player choice. Some druids will be devoted shapechangers, some will never shift forms. It's also a built in balance check, because it takes resources that could be spent elsewhere.
I am a little worried, though, that wild shape may become the default option at spell levels where it is available. It is historically a powerful effect, and I don't want other spell choices to be suboptimal in comparison.
On a related note, wild shape should drop the mechanic of replacing your character's stats with something straight out of the monster manual. Those animal entries aren't balanced for us by PCs, nor should they need to be. Instead have every spell grant a couple of bonuses to ability scores, a few skill bonuses, and maybe a movement mode or special attack. Then your form shifts to look like the animal you are trying to emulate.
For example, a low level wild shape spell might grant +2 to one physical ability score and +1 to another. You choose, so if you were shifting into a puma you might get +2 dex, +1 str while shifting to an ox would give you +2 con, +1 str. You choose from either an improved movement mode (+10 feet movement for puma), +1 natural armor (for the ox), or enhanced senses (like +4 on checks involving sight for an eagle).
It's preliminary and messy, but even at this early stage I think it's preferable to the old 3.5 style shapeshifting, with all the system mastery, supplement hunting, and game slowing that it spawned.