It's not obvious which attributes are worth buying up (pistol, hacking, lockpick, rifle) and which aren't.
The branching gameplay isn't all that branching, and its realism falls over somewhat when you act unexpectedly.
Fiddly inventory system.
Loading times between areas annoy, and move you slightly - thus leading to confusion and sometimes instantly going back to the previous area (triggerring another reload...)
Although the plot discourages you, the gameplay encourages you to kill every badguy you meet. Stealth is simply less rewarding.
Ammo constraints. ARGH!
This is the gameplay discouraging you from killing everyone ;)
More seriously, playing to avoid killing people isn't that hard. I got to the Paris Catacombes on realistic without any fatalities (except for plot dependant ones), at which point the computer deleted the savegames (mutter). You just have to spend enough augmentations on the important things, namely running really, really fast. --Angoel
Killing people gets you stuff. Often ammo. And although the ammo constraints are harsh, one bullet per person is affordable, and (almost) every person gives you a clip of ammo. It seems to enourage 'stealthily get close then efficiently take out everyone in the room' gameplay. --Vitenka
Right, onto the game itself.
It's a FirstPersonShooter. You play a secret bionic agent, who has to go out and kick butt - whilst slowly uncovering a global conspiracy and courruption within your own ranks.
Every mission has multiple means of completing it - usually a 'go in and kill everyone' option, a 'sneak past' option and a 'do something clever involving bribing people or using hacking skills' option. Then at the end, you have to decide whether or not to kill the person you meet. Repeat for next mission.
There's plenty of cool toys, security cameras, boxes, sensors and automated robots to play with. Lots of bionic and weaponry enhancements. And there's an online play mode, which lets you use all the best stuff all at once and on one another.
Any game with a 'turn on absolutely everything' hotkey has to score.
It does lack something. The environments are very similar seeming, and you get bored of industrial grey real quick.
There's also absolutely no realism. Point blank hits with rockets won't always kill, for example. But that's not really a problem.
Importantly, it's selling for a fiver on budget, and it's worth a look. Just be careful not to forget your mission objectives whilst you are investigating every nook and cranny and hacking into ATM terminals...