Overall I'm quite fond of the new design. It's definitely a more professional appearance for Scirra and will help making Construct more popular in the long run.
However, I do agree with Krush on this bit:
The style and size of the font just looks amateurish, and I have to scroll out 2 levels in Firefox just to get the forum looking decent enough to read.
The font used for displaying post content is way too big in my opinion. I also actually zoom out when I'm trying to read stuff on the forum. Sounds like a minor hiccup, but it affects usability in a big way.
Another thing that isn't optimal: when you click on reply while you aren't logged in, you'll get an error message stating that you don't have the permission to post. Would be much better if it forwarded you to the login site, like it did before.
Oh, don't know if it has been discussed before, but it appears previewing a post doesn't work yet? I'm just getting a window with a "There is nothing to preview yet" message.