Personally, I'm not quite as opposed to the new layout as some people here. In general I think it's a move in the right direction. Adopting modern standards and practices like responsive web design or AJAX is generally a good thing. What it all boils down to, however, is execution. As such, I'll try to list some of the usability and technical issues with the new layout I have noticed, hoping some of this will be taken into consideration.
First off, I'm not a big fan of the two-column sidebar on the right in desktop mode . I don't remember seeing many (if any) websites which implement such a layout. I understand this is to keep the sidebar shorter than it would've been with just one column. However, this draws attention away from the main content, especially with how much is going on in that sidebar. The root of the problem is that there are so many items. Some of them are frankly of little practical use, Random Hack Images and Random Translation Images in particular, which are just a collection of thumbnails with no other information to glean. I'd much rather see Recent Forum Posts moved to a more prominent position in the right column or even better in the left column, so I could just hide the second sidebar column entirely and be done with it.
Ideally, the site would allow you to choose what sidebar items you want to display and where, but I realize this is probably more trouble to implement than it's worth. I'd settle with each item in the sidebar having an unique ID, which can be hidden with user CSS or Adblock. Sure, you could do the same by counting child elements, but there's really no reason not to add those IDs.
As far as the mobile layout is concerned, I don't have any major grievances when using it in horizontal orientation. I think it is sufficiently functional. However, I've run into some snags in vertical orientation:
Firstly, the hamburger menu icon (show-menu) is done using an Unicode symbol which isn't guaranteed to be present in the font it's using. It displays fine on my desktop, but fails to do so on my tablet, which lead to some confusion until I clicked the blank space under the logo to display the menu. It would be safer to use a webfont like Font Awesome for this, or just pure CSS. Here's a snippet (I tried making it as close to the current symbol as I could, but feel free to tweak it):
box-shadow: 0 7px 0 lightblue, 0 14px 0 lightblue;
Speaking of mobile navigation, I personally think the Message Board should be a top-level entry instead of being hidden away under Community. The forums are probably the most often used section of RHDN, at least from my experience, so it would make sense to have it easily accessible even in mobile mode.
The forums are also a fair bit crippled on the smallest breakpoint. Since the right sidebar is hidden, you don't even get a link to view the most recent posts nor a search field. I also don't see any indication of unread private messages, either in the mobile or the full layout. Ideally, those could be displayed in the forum header under the breadcrumbs bar. Moreover, the topic listing in mobile mode doesn't even show any info about the last post in a given topic, just who started it, making it considerably less useful.
Last and not least, I would really appreciate another color scheme with less white/light-grayish backgrounds. Are there any plans for a theme that's similar to the old RHDN Pro plus the new responsive layout improvements?