With a fully open API to the backend data, it would be possible to create multiple GUI themes for users to choose from, as well as easily modify the default front end GUI or, for expert level users, even write their own custom ones (and possibly submit them for use by other site users, if they're popular). This would get around the "there's always a complaint" problem, as well as the "change noise" problem, particularly with major GUI changes.
Make the default GUI as cross-browser compatible as possible so the site works for novice users in all currently supported browsers, with some fallbacks for older browsers like IE5 and the like, but give user-submitted themes the option of limiting to browsers they work particularly well with and/or allow other users to submit modifications if needed. (These should most likely be moderated, to avoid opening the site up to vandalism and other inappropriate content concerns, but moderation will most likely not require much human effort in most cases.)
API-based Ajax/DHTML is obviously preferable for server performance reasons as well. :)
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