Dave Pollard, who often writes thought-provoking articles about collaboration and knowledge management offers a very concrete and practical set of "Standards for Intranet and Extranet Design."  There's a lot to like here, and a lot that Plone already nails out of the box.

1. Simple and intuitive user interface and architecture: Users should not require training or explanation to use the site. It should not be intimidating, nor should it require a lot of thought or practice to use it effectively.
2. Easy orientation: The entire content ‘landscape’ should be visible or at least apparent from the home page. No navigation tool or sitemap should be needed.
3. No overlap with content of the organization’s other websites: This entails knowing who the site’s ‘customers’ are, and when they should use an Intranet or Extranet versus a public Internet website. Generally, the Intranet is for employees and contractors, the Extranet is for ‘real’ customers of the organization’s goods and services, and the public Internet site is for prospective customers, alumni, prospective recruits, students, researchers, and the public media. Where there is overlap of content between these user constituencies it probably makes sense to repurpose the content for different audiences anyway.
4. Table-, macro- or CSS-driven: Changes and additions to content should not require html recoding. External websites may benefit from occasional refreshing or redesign for aesthetic or market-driven reasons, but internal site design should be driven by functionality and be changed as little and as rarely as possible.
5. ‘Bookmarkable’: Every resource should have its own unique URL that users can bookmark and find their way back to. That means no frames.
6. Expandable: The site should accommodate new individual and group web resources (e.g. blogs, wikis) without a need for redesign.
7. One-click access: Users should be able to get from the home page to the resource they seek in a single click. That may require use of menus that only show up when you hover, or scroll through lists in small windows, to prevent the home page being overwhelming.
8. ‘Taskonomy’ rather than taxonomy: In the site’s design, architecture and organization, ‘why are you looking?’ should prevail over ‘what are you looking for?’
9. Personally reconfigurable: Menus or scroll lists should be able to be personalized to accommodate each user’s browsing orientation (i.e. viewed/resorted in different ways).
10. User-driven content and tools: Content and tools offered on the site should be what users have indicated they want, rather than what ‘suppliers’ of content want the site to host. Likewise, content should be organized according to user’s needs, not the content supplier’s convenience. For example, internal news should be delivered through subscribable e-newsletters (see standard #14 below), instead of cluttering up Intranet and Extranet home pages.
11. Tools, not just content: The site should provide simple access to all the connectivity and other tools and technologies (both web-based and downloadable) that users need to perform their jobs effectively, along with online learning resources for each tool that teach users how and when to use each tool.
12. Search in context: Each search bar should only search a predefined subset of relevant content, not everything on the site that meets the search terms. The home page should therefore have different search bars for different purposes (e.g. search for people, search for documents, search for news, etc.) Nothing discourages users from ever visiting a site again more than lots of ‘false positives’ in search results.
13. Use of clickable graphics: Recognizing their higher development and maintenance cost, selective use should be made of ‘active’ graphics (e.g. organization charts, process charts) where these make finding or browsing easier or more effective.
14. Really simple publication and subscription: Sites should use RSS to allow users to ‘publish’ their content to the Intranet or Extranet, and to allow them to subscribe to a wide variety of internal and external content using a single ‘sign up’, and get that content delivered the way the user chooses (e.g. e-mail, aggregator page).
15. Accommodates different ways of finding: The site should give users three choices to find the information they’re looking for: browse, search, or subscribe.
16. Security is ‘under the hood’: Depending on your IDs and passwords (stored on your machine) users shouldn’t need extra sign-ins and log-in steps, and they shouldn’t see what they don’t and shouldn’t have access to (to avoid both temptation and resentment).

Some items that are especially thought-provoking for me when I consider them from the perspective of Plone are:

• "One-click access" (#7)-- this suggests that javascript-driven navigation portlet might be valuable.  KSS in Plone 3.0 should make that possible.
  
• The repeated emphasis on subscription (via email and RSS).  Plone does RSS 1.0 out of the box pretty well, and this suggests that PLIP 128 (Pluggable Plone Syndication) deserves some attention, and perhaps thinking more about email notification (for which we already have some support).
• "Search in context" (#12) - Plone Help Center already has its own search tool, as do various other add-on products.  It would be interesting to see if LiveSearch could be extended so that it initially searched the parent folder one was in, with a simple click to expand the scope of the search.  This might be hard to make highly usable, but it's an interesting notion for larger sites, and I've encountered the frustration on Plone.org.
• Personalizable confirability (#9).  This idea has bubbled up in the form of PLIP 152 - Dashboards.