The actual, literal form of the comment "What have you tried" is, in my opinion, bordering on being inappropriate, at least in most situations I encounter. It's not inappropriate because it asks what someone tried, but instead because it's just as vague as the question that the commentor may be seeking clarity on.
Instead, be specific. What do you mean when you ask someone what they've tried. Why do you want to know? As an op, why does this matter to me? If you explain these things in the comments, you're more likely to get the information that you need to answer the question, and the asker is more likely to understand why this information is needed, which results in better cooperation.
Hi user123, can you explain to us what you've tried so far? The best SO questions show that you've done your research, and this information also ensures that you get the best possible answers as quickly as possible and that you don't get answers that you have already tried. It also gives us a place to start in helping you. Hope this helps, good luck!
While you may not need be as long-winded as I can be, the difference between "What have you tried?" and my comment is that in this example, I'm actually communicating with the asker as a human being, which typically gets much better results for my reputation score, the asker's solved problem, his/her desire to contribute positively to the community, and the general quality of Stack Overflow questions.