Which is to say, a good user interface (and not just software, but anyplace human/device interaction occurs) should probably be as intuitive/discoverable as possible, featuring behaviours which can be inferred/suspected from an element's inherent properties (e.g.: in an aircraft cockpit, make the "flaps" switch resemble a flap), from the user's general experience in other contexts (an on-screen control that looks like a "button" should act like a "button"), or which preclude incorrect actions entirely (on a door which only swings open in one direction, install a flat plate on the "push" side and a pullable handle on the "pull" side, so the incorrect choice becomes impossible, and no signs are even needed). Putting actual instructional labels onto interface elements, whether in software (e.g. using words "Click here to..." in the text of a hyperlink) or in real life with the push/pull signs, is oftentimes a cop-out in lieu of a more thoughtful/obvious design.
In computer software interfaces (particularly web apps), I would suggest that a large, prominent, colored circle with a gloss effect and an action written/iconized on it, has the affordance of carrying out that action immediately when clicked (for some value of "immediately") -- i.e. being a button.
If substantial UI changes have taken place within an app that already has a customer base, I agree it may be valuable to bring these to the attention of the users. But a highlighted notification box appearing at the top of the app's regular UI screens, which persists until the user reads and dismisses it, is widely used -- rather than sprinkling "push" and "pull" signs around a wrongly-conceived interface[2].
Even the venerable "tool-tip" (aka HTML "title" attribute) seems often overused, becoming a crutch to support a mystery-meat navigation[3] philosophy which requires users to hover and wait to figure out what the hell _anything_ does.
[1] http://en.wikipedia.org/wiki/Affordance
[2] I'm not suggesting that the MC UI itself is wrongly-conceived or even particularly bad, just continuing with my general point.
[3] http://en.wikipedia.org/wiki/Mystery_meat_navigation
[n] The examples about cockpit design and push/pull doors come from actual published studies in ergonomics. I do not have the resources available at the moment to cite them further.