You might also want to look at Odometer: http://github.hubspot.com/odometer/docs/welcome/
odometer(el, 42);Odometer is nifty in this way because you can still just set the value with innerHTML or .html, and it will animate it, so the overhead of adding it becomes just adding the class to whatever elements you'd like.
You can actually also manually instantiate one pretty much exactly as you described:
new Odometer({el: el, value: 42});
One pattern I picked up from reading TJ's code is the transparent constructor...
odometer(el).value(42);
function Odometer (el) {
if (!(this instanceof Odometer)) return new Odometer(el);
...
}
I agree with wrapping the constructor function. Requiring the consumer to remember new adds one more tripping point for users. In this case, I suppose my thought was that the manual-creation path was such a small percentage of users that it wasn't a huge deal.