Pre-order bonuses are generally pathetic offerings, like a weapon skin or a t-shirt. However, Eidos Montreal is making the pre-order exclusive for Thief very sizable. Anyone who reserves the game will get access to a new mission called the Bank Heist.
In this mission, master thief Garrett must infiltrate the Stonemarket First Bank. The building is patrolled by armed guards and outfitted with a number of other security measures you won't find elsewhere in the game. The bank is thought to be impregnable but that's never stopped Garrett, has it?
Bank Heist is inspired by the First City Bank and Trust mission from Thief 2: The Metal Age. The Thief 2 mission had players dodging security cameras and robots. I didn't see any robots in the trailer for Bank Heist but the video did seem to show a couple security cameras. In Thief 2, those cams definitely complicated things; they set off alarms if they spot Garrett and can't be disposed of very easily.
By cracking the bank's security, you'll get access to loads of loot. The trailer mentions that your completion time and haul from the mission will be measured against your friends. It's not clear whether this mission is just a standalone challenge or whether you'll be able to keep the money you earn and use it in the campaign.
It sounds like an impressive bonus for simply pre-ordering the game. That's a double-edged sword, though. On the one hand, it's nice that people who gamble on the pre-order get something nice in return. On the other, Eidos is walling off in-game content from players who don't want to pay money for games before they've read reviews. These players will presumably have the option of buying the mission after launch but that doesn't make the whole thing any less unseemly.
Thief will launch on February 25th in North America and on February 28th in other territories throughout the world. You'll be able to purchase the game on PlayStation 4, Xbox One, PlayStation 3, Xbox 360 or PC.