Oxygen skin makes too little use of wider screens and is equally bad on hand-helds. Chihuahua is now default and uses all sizes correctly (fingers crossed)
Languages using more space for translations, will mess up the tabs (solving former, will solve this in some cases). Try Dutch compared to English. The last tab will be displayed just below the tab before that. Using icons instead of text, to avoid this problem
Right to left languages, alignments, etc. There are open issues for this in MediaWiki's bugzilla. Ask Siebrand for more details if relevant.
For documentation we need to work towards notification of 'more recent version than last exported to distro'. Means that revisions should be taggable with definable tags and trigger a certain (localisable) message to be displayed for page translation pages. (lower prio)
Consider activating the Usability toolbar. Consider using the same features as in translatewiki.net, or possible be a little more conservative because this wiki's functionality should be rock solid wherever possible.
LiquidThreads uses a little older version; trunk is much better, but often unstable revs are put in there. Might want to check with translatewiki.net if there's a stable version, and then update to a particular rev. The version used on UserBase at the moment is a little old and messy particularly in display.
MediaWiki allows sending out RecentChanges to IRC. Ask Niklas for details if you want this at some point in time. Requires a relay agent.
Analyse current content, find out what is English, and needs to be translated
Tag outdated content (or move to a different namespace) - create a process for this - or at least document it?
How to improve they way in/how to allow users to find what they need as soon as possible.
Make search results language dependent (is new to be developed functionality - should probably be its own extension before giving it to the search engine)
Don't put each language in its own namespace. It'll be a nightmare, because there are too many languages. In the beginning it'll look like a solution. Imagine having 150 namespaces when supporting 67 languages (2x67 because of talk and the 16 or so default ones).
set up a Translation Memory server - or use the one from translatewiki so repeating sentences/words will be suggested to translators. Use Apertium, too.
consider installing the Babel extension. Siebrand already created in-wiki settings, needs to be added in LocalSettings.php.