Well, the old technical limitations of the web have muddied the affordances somewhat, but my rules is generally "add cursor:pointer if the button behaves like a link, leave it unchanged if the button behaves like a button".
Most of the G+ buttons actually behave like buttons, i.e. you click on them and something happens on the current page. They don't have cursor:pointer, but they have other standard button hover effects. Pre-AJAX, the web had a history of tarting links up to look like buttons so they would seem more "clickable". Those kinds of "buttons" should get cursor:pointer, but not any other hover effects.
Agreed completely. At minimum, and this should be obvious, but you should never leave the text/I-beam cursor on something that can be interacted with (e.g. buttons, sliders, tabs). (I would have only upvoted you, but with hidden scores, who does that help?)
Most of the G+ buttons actually behave like buttons, i.e. you click on them and something happens on the current page. They don't have cursor:pointer, but they have other standard button hover effects. Pre-AJAX, the web had a history of tarting links up to look like buttons so they would seem more "clickable". Those kinds of "buttons" should get cursor:pointer, but not any other hover effects.