Java (J2RE / J2ME) to expand the app capability of the device. eg Opera Mini

Persistent web cache in the web browser available after the browser has been closed and restarted, so regular pages don't take up unnecessary bandwidth downloading images again. Useful for people on a limited bandwidth mobile tarriff.

Quick option in web browser to enable / cache / disable images.

Theme Edior / manager to allow easier changes to system icons, colour schemes and fonts/font sizes. this is especially useful in relation to apps which need a smaller font to make better use of the desktop space (ftp / irc). Maybe even allow changes to the size of the window title bars allowing for scalable icons

Portrait mode optional. I know some apps are hard coded as portrait, but system views could be supported. Only issue i could see is that each desktop would need another layout saved for the position of the widgets in portrait mode

The purpose of theme edting and portrait mode is to let people optimise their experience. For example switch between primarily one-handed with thumb operation if they are on the move (larger icons / fonts) and two-handed / desktop operation with stylus (smaller fonts / icons). Or through TVOut (even smaller fonts / icons / windows).
All of this to customise the experience based on what they are doing and what the needs are at time, all of which could be achieved through customisable themes on the device.
Three main layouts for casual / non-power users, but all customisable for power users to save as separate themes.