Nice integration between the forum and website!
-the text font (specially in the forums) seems too big and get a little uncomfortable to read;
-the page's content container seems a bit too narrow. As the most common resolution nowadays is 1024x768 or higher, I think it could be at least 900px wide to optimize screen space;
-if implemented the above suggestion I recommend enlarging a bit the lateral padding area as well to let the content breathe a little more;
-in the topics table would be nice to give odd rows a slightly distinct color to improve readability and association among the information on other columns;
-it would be nice to slice big tutorials into multiple pages. Maybe a system similar to instructables.com where you can separate into steps, visualize them individually or all at the same time.
This is more a matter of personal taste, but I particularly don't like the two tones of green together as to me they give a too eco-business identity. I think a gray would suit better instead of the dark green, and also be more techy.
quick edit: http://dl.dropbox.com/u/7871870/construct/scirra_website_01.jpg
Although I think would be even better if the header called a little more attention to it:
quick edit 2: http://dl.dropbox.com/u/7871870/construct/scirra_website_02.jpg
Use them as you wish.