Warning: There are (minor) spoilers about Deus Ex: Human Revolution in this review, so proceed with caution if you've yet to finish the main game.
The first piece of DLC for Deus Ex: Human Revolution wipes the slate clean. It's an opportunity for players who were disciplined in a certain style of play to acquaint themselves with another. For those who passed through the game as a shadow, it's a chance to crack out the guns and start causing a ruckus. For those that killed everything in sight, it's a chance to take it down a notch and try out the more elegant stealth side of the game. Or not. Whatever. You could choose to play the DLC in exactly the same way as you did before. Nobody's going to make you do anything you don't want to. Such is the beauty of Deus Ex.
The Missing Link is named well, taking place in that hazy gap between Hengsha and Singapore. Jensen wakes from stasis and finds himself sat trapped in an EMP chair, which renders all of his augmentations little more than dead metal. His captors - Belltower agents - smack him around for a bit before bombarding him with questions. Ever the tough guy, Jensen keeps shtum. The interrogation scene lasts little more than three questions, and could have been drawn out much longer. That said, your choices here do affect what equipment you'll recover later on in the game. Choose an aggressive response, for example, and you'll be rewarded with guns.
After being left unsupervised, the shackles on his wrists and feet are released, and Jensen is able to make his escape. Through a ventilation shaft, obviously. The Missing Link plays out like any other mission after this point. It's longer, though - weighing in at four or five hours, less if you're good - and self-contained in terms of plot. Light is shed on some aspects of the main story, particularly Megan Reed's research, but the DLC has its own plot and themes.
In terms of what you're actually doing, the formula is familiar: you'll move from objective to objective, taking out (or strategically ignoring) guards along the way. Some new weapons or augmentations might have mixed things up in this respect, but in their absence the game plays much as you'd expect.
The environments are new, of course. The boat is more claustrophobic than previous areas, and benefits from a tweaked aesthetic and improved lighting. The area you'll prowl about is littered with short cuts and alternative routes. The level design promotes a stealthy approach, which is reinforced by the voice - an unknown accomplice acting as your handler - jabbering on down the comms system; they'd much prefer you to be quiet.