The option to add a class to a button was added in 3.3.4.

@Ant Forshaw in case you would like to vote for other elements, It's probably a nice idea to create a new request post, since this one only indicates a button.

Adding classes to ANY element would be great, because not having the ability to add a class to a text-box is a big deficiency.

Great idea! It will be so useful for me too...

Yes that is a good idea !

We need it too.