Like all forms of art, this game represents the woes of the modern society.
As far as the game style goes - nothing changed much - the gadgets are still the same (except a few new grenades and some advanced MK2 models of the old stuff), the sneaking around and using the environment to your advantage is unchanged. However the big change is the system of detection - from the light and sound system we had before it has been changed to just 3 levels of exposure - hidden, exposed and detected. The lockpicking is back and is pretty straightforward, except the digital one used for opening saves which can be a little tricky but you get the hang of it eventually. The learning curve as a whole is about 30 minutes - 1 hour but that's mostly due to controls. For veteran players this port is already known and feels OK, for new players it might be confusing but by the end of the game you can consider yourself a splinter cell veteran since this game incorporates everything from the old stuff and cleans it up a bit, gives it a polish and a nice new look.
Other mini-games are all over the game, one lvel is a compound to where you return all the time and there's loads of stuff to do there (you have infiltrated a terrorist organization so you walk around putting bugs everywhere, viruses, leaking data, fetching intel etc.. all while the terrorists think you're one of them). The player is consistently faced with numerous choices and morale dilemmas which can backfire and influence how the game flows. To win the game you have to juggle with both spectres of the human codex. The game, ergo, has several endings, depending on how you play your cards. Double Agent also gives you a chance to assemble mines, disarm bombs, scan fingerprints and retina of individuals and record voices (all these to unlock doors), you can also hack into terminals, doors, bash doors open, use optic cables to check what's on the other side, break ice under water and grab enemies in order to pull them down etc..
The game can be completed without shooting a single bullet (the proper stealth mode - the way it's meant to be played) or you can go out all guns blazing (however - this way you are more likely to fail objectives and trust, not talking about the unlocking of different new gadgets).
The graphics are neat and you can play on max levels on a modern machine. The AI can be smart (when it's light) and really blind and stupid (when it's dark).
I wouldn't compare this game to other Splinter Cell games since I love them all and played them all for years. This one is just as great and just as fun and adds a nice bunch of new stuff. So, unlike Hitman, this is still fresh and feels like home.
After all, it's still Red Storm, 13 years on.