It's the most original forum "moockup" i've ever seen. It's something of the level i wanted to do for a website of mine for years now.
So, i'm positively surprised.

But again, you shouldn't start fresh out of nothing.

Not exactly a fan of the mock up UI. https://inputs.io/beacon Individual posts take up too much space, and black on grey is not a good color combination for long reads. The

or whatever makes the subject take up disproportionally more space - even through in 99.5% of the posts the subject of replies convey no additional information.

The topic list UI is not well designed. The user bar should have a constant width that makes it easier to skim through the subjects. There's too much clutter to the right that makes it hard to see replies / views at a glance, and there is no last reply message. The timestamp is also unfriendly - you should use relative time (eg 3 minutes ago, not dates unless it is very old).

Most people also generally read from left to right. IMO the sidebar would be better positioned on the left (and you should really get another color palette).

Quote

The default theme should be minimalistic like the current theme. Nothing that looks "web 2.0": no speech bubbles, no significant space between posts, no significant hover effects, and few rounded corners.

The default theme must work well with all functionality and a reasonable page layout on text browsers without JavaScript. It should also work perfectly on browsers with unusually small browser dimensions. It should be at least somewhat usable (though maybe not pretty) on ancient and broken browsers like IE6.

Same color scheme as we have now: light with some blue.

I also recommend to not go with major design overhauls to make it look web 2.0. I use Bootstrap a lot, but it's simply not what will work with the Bitcoin community. That means, don't start your project off with Bootstrap's large empty spacing, font size, large buttons, white/grey/black palette.

Making something more web 2.0 has disastrous results for a technical internet community. DigitalPoint migrated to the latest version of vBulletin which is like the direction your UI is going. Take a look at how they are doing:

http://traffic.alexa.com/graph?w=340&h=150&o=flt&c=1&y=t&b=ffffff&n=666666&r=2y&f=999999&u=digitalpoint.com

You can quite easily see that after upgrading their forum to be more web 2.0 / shiny, their userbase stopped visiting as much. They had to move to another forum software that was more traditional, and better for "power users".