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 default color palette).

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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.

Keep in mind that the audience here is much more technical, and are more power users. In addition, a much more significant amount of users runs with JS disabled or NoScript for security reasons (against code injection, using tor, whatever), your user interface should be virtually identical (like right now) for users without JS enabled. This reminds me of the disastrous Google Reader redesign - too much empty space, too much user friendlyless at the sacrifice of user functionality - or digitalpoint (that's a way better example).

Many users (especially power users) do not have an avatar here. How will that be presented?