What were the reasons the reviewer gave? My first thought is that a different overall background would make a world of difference. The current one is distracts from the rest of the design. I would try something a bit more subtle and lighter.
Another thing you should toy around with is the H tag font. The one you have is cool but it becomes kind of hard to read at times.
I would also expand out the footer a bit. There’s nothing wrong with a minimal footer but if you make it a bit taller and spruce it up it will help add some content to the page templates that don’t have a lot of content (like the Clients page). You may also want to add some extra content to some of those templates, even if it’s just some dummy text.
One final thought would be to experiment with adding some subtle use of color. Everything is very very grey right now, and while I think grey is a great dominant color some extra splashes of bright colors like orange, teal, red, etc could go a long way.
I think you’re definitely on to something here, but those are some areas I would toy around with. Good luck!