Per the post at http://wordpress.org/development/2009/12/setting-scope/ we will be attempting to set a defined scope for version 3.0, and stick to it. Use this thread to discuss features for inclusion, and which features you think are most important to have in core vs. a plugin. Bear in mind that version 3.0 is when MU functionality will come into core, so WordPress will support running multiple blogs/sites from one installation, and that merge is in itself a big project. If you could only pick *one* other feature, what would it be?
Here are some of the ideas that have been discussed for 3.0 before... the list is clearly too long, but it will give you an idea of what's been proposed or pushed before:
Easy blog menu management, dynamic image resize/crop, media upload UI redesign (begun in 2.9 but postponed for implementation due to technical issues), photo albums, custom content type UI and API, supercharging queries (cross-taxonomies), categories/tags for pages, auto-taxonomy UI, custom fields UI (possibly to be registered by themes or plugins for something to be displayed), settings UI redesign, improve the upgrade process (inc. distros for specific use types), SVN awareness, canonical plugins and a UI for displaying them, plugin page redesign, themes UI redesign, comments UI touchup, decouple language updates and files, new default theme, choose your own start page, caps lock detection, accessibility admin theme, mobile admin theme, synching custom fields > taxonomies, exif refresh, role management simplification, credits page in app, default custom types (microblog, galleries, asides), admin bar, front end comment moderation, front end posting (a la P2), better importers, widget installer, importer installer, more inline documentation, built-in "Welcome to WordPress" guide for 1st time admin use/checklist (set settings, add profiles, set up comment options, dashboard modules, add widgets, pick a theme, etc) with ability to dismiss as you move through, better help tab, more template tags, better zone selector, new code editor, XSL for RSS feeds (pretty feeds), bulk user creation (lazy load importer?), below post widgets, image importing, HTML validation, customizable comment form, Twitter and Flickr importers, WordPress capitalization catcher, configurable QuickPress configurable (add categories), more dashboard modules, easy linking to internal content when writing new content in editor, audit of error messages and updating them to be clearer revisions for custom fields and taxonomies... the list is endless, really, because there are so many cool things we could do. But which ones *should* we do? And specifically, which should we do in 3.0? Discuss!