We've just rolled out some changes to zooming. This was to address a couple of issues:
On desktop browsers when zooming in and out using the mouse scroll wheel the map sometimes seemed to overzoom and then "bounce" back one level, which was slightly disturbing. We've now got this under control.
On touch screen browsers (i.e. mobile phones and tablets) pinch zooming no longer worked. We've fixed this.
We've also done some tweaks to the interface for touch screens to make the "+/-" zoom buttons bigger, and so easier to use, and also de-cluttered the top bar to remove some boxes which dan't (currently) work with touch screens.
P.S. If you find that the changes for touch screen browsers aren't working for you then please visit this page from the browser which isn't working and then post here the result.