I've been playing your game since July 2015. I love this game. However, I cannot hide all of the game's UI. There have been many missed screenshot opportunities because of this.
Currently this is the only way you can get a clean HUD (the compass cannot be hidden)
• Options > Interface > Quest Tracker
• Options > Nameplates > Nameplates
• Options > Nameplates > Healthbars
• Options > Social > HUD Chat Display
• Options > Combat > Ability Bar
• Options > Combat > Attribute Bars
• Options > Combat > Buffs & Debuffs
Look at how many options you have to go through to get a clean UI =D