I looked up some more details on this and found some of the more interesting changes to the Director's Cut, summarized below:
New Boss Fights
• Boss fights have been entirely redesigned; new map layout and new gameplay mechanics that support the game’s main pillars (hacking, stealth, combat). Also feature AI, damage & health tweaks for the best boss fights possible.
• It’s now possible to defeat bosses without the need to fire a single bullet.
This is the big one, of course. Sounds like they're making the fights what they should have been to begin with.
Technical & Other Improvements
• Rebalanced energy system; Able to use more augmentations with more energy cell recharging. Makes the game more enjoyable and accessible on lower difficulty levels.
This is also nice. The original implementation of the energy system was kind of weird. I spent most of the game with only one energy available, which is kind of counter-intuitive. I'm looking forward to seeing what kind of alternative they came up with.
• Improved visuals: With a better lighting system, fog and smoother shadows; all visual improvements from The Missing Link have been applied through the whole game. And with anti-aliasing, this is the best looking Deus Ex available.
Better graphics is always a plus.
• AI tweaked for consistency
Whatever that means.
• Director’s commentaries; For the hardcore Deus Ex fans, approximately 8 hours of director’s commentary that reveals secrets and background info on the creation and production of Deus Ex: Human Revolution – Director’s Cut.
• Strategy Guide: A strategy guide is available for the complete game including both the Missing Link and Tong’s Rescue mission. The strategy guide is implemented in-game and can be accessed through the Neural Hub at any time.
• Achievements system; Original game achievements, The Missing Link achievements and new achievements are available!
• A “Making Of” Video; The “Making Of” video of Deus Ex: Human Revolution, approximately 45min.
• Left-Handed control mapping; for a total of 4 control layout variations
• Languages: English, French, Italian, German and Spanish are available in both voice and subtitles. Choose to play the game with audio in one language but keep the subtitles on another.
This is cool. I'm going to watch/listen to most of this, I expect.
• The “in-game” menus are now toolkits; they feel like part of the game flow and one of Adam Jensen’s augmentations.
• Tongs’s Rescue mission and the entire Missing Link chapter have been integrated seamlessly into the narrative flow of the Director’s Cut.
• Original pre-order and other bonus weapons are now at Jensen’s disposal.
All good news. Particularly the bit about integrating the DLC with the original game.
Most of the rest of the stuff I found was Wii U specific, so I won't bother listing it. If you want to read the whole thing, you can find it here