For those who don't read the update notes and/or don't see the sticky thread at the top of the page (myself included until recently), pressing "T" doesn't toggle the Quest Arrow on and off anymore.
The new hotkey is CTRL+T (hold down the "Ctrl" button and press "T").
According to KingsIsle, the reason for the change was to prevent players from accidentally turning the arrow off (or on). This makes sense, since under the old system, it was very easy to type chat while the chat window isn't open or in focus, and turn the arrow off.
Of course, the Quest arrow can still be turned on and off through Preferences. It's the fourth tab from the left, the option at the bottom.
