1. The background looked kind of cheesy. It's not a huge deal, but a solid color would look nicer.
2. There's a timer the second I load the page. If I'm trying to relax with Sudoku, this site is not for me. It looks like you can turn it off, but I know people who will just leave once they see the timer. Timed games are pressure-inducing and not fun for them.
3. I don't know what the various tools shown do. Why would I need a pencil tool for sudoku on the computer? Can't I just type the number I want? What's the one that looks like a checkbox? Why is there an error check? Other sudoku apps just mark it in red if it's wrong. (These are things I would think as a normal user. Obviously I read the tooltips, but most users will not.)
Overall impression - hard to use, not fun. Plus, a little late to the Sudoku trend. There are thousands of Sudoku sites and mobile apps. What would be my incentive to use yours? If I already have one I like, why would I switch? Also, I have ad blocking turned on, but if there were ads and I saw them I would never come back to your site. (Obviously different people have different feelings about that.) I can pay 99 cents for a great Sudoku app on my phone with no ads.