First of all, Nornia isn't a new theme. It's an old theme that was deleted from the 1.9 branch but was overlooked for removal from 2.0. I'd stay away from that one.
New to the bundle are Darkroom, Feb12, The News, and Tikinewt. These all use the *litecss method for layout (divs only for main page layout, etc.) and so are more modern in that sense. There'll be more info on modifying *litecss themes soon (I hope), but for now just be aware that column widths and margins are sensitive and must stay in synch with each other. This is indicated in the CSS file for the theme. These themes, having no main tables to "sandbox" stray div tags, are vulnerable to breaking up like a cubist painting if there is an extra /div tag in the wikitext or somewhere, so care is needed in that way.
Also look at the alternative themes for 2.0: absE, Andreas08, Andreas09, Fluid Index, CandiiClouds, Faulkner, Kubrick, and so on. There are download links on the New_themes page. Of these, some — the last three listed for example, use a table for layout, mainly because IE7 doesn't cooperate with a centered box loading content of wide sizes, etc., using divs only.
All the themes I listed were made new or updated for Tiki 2, so should provide good starting points for customizing.