I'm intrigued by your question. The toughest CS problems are challenging for the most experienced CS researchers. If you are one of them, then why do you ask?

I'm intrigued by your question. The toughest CS problems are challenging for the most experienced CS researchers. If you are one of them, then why do you ask? If you are not, then how do you plan to beat them at their game? The best snapshot would be to read the conference proceedings from SPLASH 2013. Then you could read the last 12 month's transactions and conference papers put out by the IEEE and ACM. These sources should give you a clear picture of what is leading edge research and what the open questions are. Good luck! and keep us on HN posted about your work.