[Groxx]

To simultaneously answer many of the "usually you don't want X to be selected":

So don't always allow selecting everything. If you start your selection inside "content", constrain to content, don't select labels. If you start on a label, select labels and maybe content too.

Native apps do this all the time in small degrees, people are used to it: "select all" selects content, not chrome, and it's context-sensitive in many cases (e.g. select all in a folder doesn't select parent folders even if they're visible).

Don't block selection please.

[ubercow13]

I don't want to select text in a button

[Groxx]

having encountered some buttons with significant amounts of text: nothing is off-limits IMO. frequently tho, yea, "ok" isn't useful to allow to be selected.

On the other hand, getting people to copy what they see rather than read it out loud incorrectly can, pretty often, cut several rounds of back-and-forth out of remote tech support. It seems insane because it is, but yes - copying text in a button rather than having them say "I clicked the yes button" (when no such button exists) is a useful feature.