When you try to edit a question, there is a request to not make trivial edits. I wonder what counts as trivial? Adding better tags? Expanding an abbreviation in the question title? Enabling syntax highlighting?
Fundamentally edits should improve a post. However, minor typos - things that don't change the context of the post - should really be avoided.
I think the biggest reason behind this stems from the fact that an edited post becomes active, and active posts get bumped to the front page. In that case, chaning "teh" to "the" or removing "Thanks!" doesn't really help, since no new value has been added.
On the other hand, I've made minor edits to post if they've just been bumped as there is no real harm in doing that to an already-bumped post. I consider those just edits missed by the other person.
A broad discussion on (in-)acceptable edits is available from When is (and isn't) it acceptable to edit?.