While everyone here is focussing on the front end theme, I'd also like to throw thoughts in about the admin theme.
Basically, I hate it.
Since WP 2.5 the admin side has been awful - it's way too slow to load as a page, and opening a sub window, such as the media gallery, fries my CPU and takes forever to load - this has been an increasing problem in every sub-version of WP since 2.5 was released.
Lighten up the admin theme, make it load and run as fast as the front end. Just as an example, I'm a fairly fast typist, and there are times when I've written something and realise I wrote it wrong, then go to backspace delete and look to the screen to see how far back I have to delete, but the screen is still "writing" what I've just typed. Trying to backspace delete over it renders a one character movement per second or longer. This does not happen in the front end comment box, nor on other sites such as here in the WP forums - ergo, it is a problem with the admin theme.
Another factor that makes me hate the current admin theme is all the widgetised stuff throughout - I want to set my admin and leave it, losing the bandwidth, page-code, and rendering overhead of all that java (or whatever it is) so that I can work FAST, not be always waiting for wp-admin to catch up with my keyboard or mouse.
Right now, the biggest offput with WordPress is the "slug on Prozac" symptoms of the admin interface. I know some of you will say it runs fine for you - but what computer spec are you running? If WP needs a dual-socket quad core Intel 3Ghz+ with 16Gb RAM and SAS drives ON THE WORKSTATION PC, with 16GB+ internet connection, to get tolerable performance from the admin screens, then not only should that be stated in the pre-install documentation, but the developers need a good hard slap around the head for not thinking about the planet's majority of users who are still running 32-bit computers with single CPU, 512Kb RAM (or less), with 8Mb graphics cards and sub-100Gb older and slower hard drives, running sub-2GB internet connections.
The connection and hardware specs needed to get wp-admin to run at reasonable performance is marginalising WordPress outside the north-western quadrisphere of wealthy-nation propellor-heads - think about this BEFORE releasing WP v3.0 - PLEASE!
Regarding the revised theme for the site's public side - my requests are simple and already stated by others -
- child themes of similar nature to BuddyPress, but easier to understand and implement, with compatibility with BuddyPress regardless of whether originating in WP or WPMU.
- heavily commented css and php files so users and developers know what each part of the file is attempting to do.
- easily identified and compulsory options for fixed width or fluid (not all theme layouts are compatible with fluid width when the user has a "small" monitor).
- FIVE default sidebars pre-defined and included - header, footer, each side, plus a horizontal "in-content" sidebar.
- multiple page and post templates (e.g. with / without sidebars, single & multi-column) to show this is a major feature of WP.
- both home.php and index.php as standard files in the default theme (they're needed for WPMU and BuddyPress anyway, so why not include them in WP standalone?)
- post auto-thumbnails, with thumbnail size either settable on the fly when posting (custom fields?) or definable from a post-page panel.
- more "wrapper" divs as standard for background graphics and colours (I know how to do this myself, but how many newbies do?) with a simple options page in admin and the css pulling the data from that.
I could go on, but many of my "wants" are certainly in the plugins or theme customisations arena, and should not be in core code.