But are all tropes bad? Is it ever okay to use one?
I think there are at least two times when a trope can work for your story. For instance:
When the trope is very broad
Some tropes aren't very specific and won't give your readers too many spoilers about what's going to happen in the story, like the unlikely friendship trope I posted about not long ago. Your readers will probably expect certain things from the story, such as the inevitable point when the two "enemies" become friends, but there's plenty of room for variation within the loose structure of the unlikely friendship idea.
On the other hand, the "chosen one" trope is far too specific. It's about an "ordinary" person who is singled out by fate, decree, prophecy, or some other impersonal means as the hero the world needs. Once you know that an author is using the chosen one trope, you can be pretty sure that the protagonist will discover that he is "special", will probably be endued with remarkable abilities, and will spend a fair portion of the story in angsty diatribes against his fate.
But even annoying, overused tropes can be pulled off if you think outside the box a bit. For instance,
When you twist the trope
Since readers are already expecting certain things from the story because you used a common trope, doing something that isn't usually done in the trope will come as a surprise (usually a welcome one).
(Gabrielle @ Write for the King just wrote a great post about five ways to twist the Chosen One trope.)