You can read up on what the addon API supports in the forums. Last time I checked it was _extremely_ limited. They're aiming for the "small scope and good quality" approach rather than "a lot of features but everything is broken" for initial version.

However, I'm reading the patch notes now and there's a lot of goodies there. Besides the big ticket items, I like these ones:

  1. You can now add [notactive] to a /cast macro to prevent it from turning off a toggled ability.
  2. Parties can now be displayed by default using the Raid Frame UI. Check the Display tab under Options to enable this setting.
  3. If you are in a raid with less than 6 people, Prestige and Favor awards will be calculated using the party values.
  4. PvP deaths no longer result in soul damage, even if NPCs are attacking you.
  5. Cleric Cabalist: All Sigils are now area of effect on the target rather than ground targeted area effects.
  6. Added a keybind-able command that will hide other players on your screen when the key is pressed. There is no default bound key, but you can set one in the Key Bindings menu as 'Hide Other Players' under UI Toggles.
  7. When right-clicking stackable items to move them into a Guild Bank, they will be merged into any existing stacks in the bank if there is room.
  8. Quests that share updates with party members will now also share across the whole raid, not just within your raid group.
  9. After invaders take over a location they will start stacking the 'Burning Out' debuff, causing them to take increased damage for each stack.
  10. Area effect heals that were using up a heal count on a dead friendly target will no longer do so.
  11. The Marksman 'Bull's Eye' and other similar abilities will now work properly in macros.
  12. The Auction House now has a new 'Vanity' subsection to look for wardrobe items!
  13. General client performance tweaks.