Feature number one on my list is the new "floating combat text" functionality. You'll find options for it under the interface options menu (see right), and it mimics the Scrolling Combat Text addon, which shows combat information floating over your character. I've always found this to be a more convenient place to view information, but it can be too spammy at times. And, if you've never played with Scrolling Combat Text, I'd recommend giving it a try now that it can be enabled without downloading anything.
The look of the addition is dead-on, though it's a bit less customizable than the real thing, which allows you to select more precisely which messages you'd like to see, as well as customizing colors and fonts. This simplified interface may be more accessible to more users, however, as the number of options Scrolling Combat Text provided could be a bit daunting. However, I may end up sticking with the original, because I prefer to be able to resize the font used.
Of special interest to me as a healer is the new name plate functionality. If you aren't familiar with the phrase, name plates are the little health bars that show up over enemy and friendly NPCs when you hit the "V" key. In the past, name plates would only appear above hostile targets and NPCs. It's long been on my wish-list of features to have the ability to have name plates over friendly targets as well - for ease of seeing the health of players around me when either ungrouped or in a scattered raid, where you may not necessarily be near your own party. (Battlegrounds, anyone?)
Now, if you check your key bindings (see above), you will see a binding for showing name plates (for the existing functionality) as well as one that allows you to show friendly name plates or all name plates. Combined with the battlegrounds change that automatically groups players on joining, and my priest may have to respec holy and go back to trying to heal in battlegrounds.
Next up is a feature that came as a complete surprise to all of us - iTunes integration. This is a Mac-only feature, as, apparently, the Mac development team managed to get it working first. If you're running the Mac client, you'll see the above options at the bottom of the list of key bindings, allowing you to bind keys to basic iTunes functions.
The final feature addition, at least that I noticed, is the new auto self cast checkbox in the interface option. I've used a feature like this before - as part of CTMod, and probably other addons as well. With this option checked, if you do not have anyone targeted when you try to cast a friendly spell (a buff or heal spell, for example), it will automatically be cast on yourself. In some situations, and for some character classes, this can be very useful - allowing you to keep a hostile target selected while casting useful spells on yourself. But it can be difficult to get used to, especially for healing classes who frequently use little but friendly spells.