We've already discussed the combat log changes somewhat, talking about how users can set up filters. Another change in the log is that there are now four tabs to it, called Default, Everything, Me, and Friends. That should help to cut down on log overload, although honestly, I ditched the combat log for various floating combat text mods long ago (currently using and loving MSBT).
Additionally, the crufty Interface Options window has been renovated and categorized, much like the keyboard bindings interface was a few patches ago. This should make it much easier to toggle your helm off and on in order to confuse your guildmates. Looking through all the option screens (see Blizzplanet for screenshots), I see a few I don't recognize. "Detailed Loot Information": Anyone know what this means, and if it was in the game before? Also, I don't remember "Show free bag space" being a part of the default UI. Anyway, good improvements.