As Doomex suggested, highly recommend you study the wiki. Also check the patch notes for all the changes and added features. There is so much new content it would take to long for anyone to type it up here!

As for gold, it is only obtained through voting, donating, or event rewards. You don't need gold to repair, hire mercs, or buy potions (it's all free). Basically gold is used for buying items from the Hadriel NPC.

Here's a quick list to get your started:
1. Tons of new items
2. Monsters are harder than normal.
3. Many skills have been buffed/reworked.
4. All uniques/sets/rares are buffed.
5. Many new maps have been added.
6. Max players in a game is increased to 64.
7. Each character has 1 new skill.
8. Observe/spectate feature has been added which gives you the ability to watch other players in the game. You can gain experience and quests while spectating.