Wow, you guys are a tough crowd.
While it certainly might be useful in italki to ferret out those who might be ungrateful users of the system, generally speaking "thanks in advance" or the more formal "thanking you in advance" should not be read as pushy, ungrateful or obnoxious. In business letters I have always felt it a better ending than "Sincerely yours", and felt the phrase more akin to "thank you for your consideration" or "thank you for your time".
I really don't understand the harsh reaction here and I do not think it an impolite phrase in general use.
Whenever I see the phrase"Thanks in advance" in the notebook section, I feel awkward. The meaning might be "Pleased do it" though, I prefer "I'd be glad if you corrected my notebook." When my boss texts me and asks some tasks, he always finishes with "Thanks in advance", I feel I have to do it immediately. I don't think it is rude or wrong but you could think about what impression you give to others. Polite way is much better than a casual way when you ask something.
I think various reactions happen in culture difference, so if there were people who don't like it unlike you, you could use alternative phrases to improve international communication. "Thanks in advance" is easy and handy to say. But I'm wondering if the expression of appreciation could be measured by how much convenient it could be.