The responsive system is decent but one thing would be a major improvement, and that's the menu. Here's a few things I think would be excellent.
1. Make child pages expand the menu (menu > submenu). Currently all pages just show up in one big list, fine if you have a 6 page site, but for larger sites with multiple child pages it's a mess. Make each page with child pages have an arrow/icon > so people click to see the child pages.
2.A separate Header image for Responsive . A header that works for the normal page view often is a mess (showing blank space instead of any of the actual header image) on responsive so allow for different header on the responsive views, so a smaller, better fit header would be possible.