Just for clarity I played the PC version on highest difficulty with highlights and cover notifications turned off and cover mode set to toggle. I also realise now that I played through the entire game with the targeting reticle turned off. I didn't even realise I turned it off, but I would actual recommend it. It adds another bit of challenge, makes the iron sight aiming more useful and the laser sight invaluable.
This game is very good. Lets start with that. Everything works very well. I thought the cover system and jumping between first and third person constantly would annoy be but it didn't at all. The cover system is smooth. Transitions around corners and between different covers works perfectly. I have no complaints about it. Similarly for the takedowns. At first the idea seemed stupid but to be honest they're just fun. Many of them are pretty funny or just cool (especially the double takedowns) and the way it uses power prevents spamming them. A much more fun system than standard melee attacks.
Regenerating health is thankfully not the wait five seconds for near-instant recovery sort and it actually takes some time to heal you back from the bring. Not something you want to be relying on in the middle of a fight, especially against tougher enemies. It does mean you don't have to run around looking for med kit if you screwed up and used all of yours and have 15 torso health as happened to many first-time Deus Ex players and it's worth noting that if you use health items in Human Revolution you can bring yourself up to 200 health while you'll only regenerate to 100, meaning regeneration only bring you to half your effective total health.
I don't want to go into detail about stealth or shooting gameplay. They're both good and both provide a decent challenge. A higher difficulty setting might have been nice but for "Give me Deus Ex" gave me a fun challenge for a first playthrough.
Hacking is great fun. Certainly the best hacking mini-game I've played in a game. Blows Fallout 3 and Bioshock out of the water. I hacked most stuff I came across (except the alarm panels, too many of those to bother with) and the emails on the PCs are sometimes pretty interesting and sometime quiet funny.
Exploration is cool and rewarding and the city areas are pretty lively and well-populated. Unfortunately very few characters have unique interactions though. You can overhear a lot of conversations but you can't go up to the people in question and talk to them about whatever they were discussing. There are no small one-objective sidequests for random people. You can't question the civilian population for info on places you're going into. The lack of significant interaction with small NPCs is unfortunate but it is made up for by the great interaction you have with the important ones. The dialogue system is very good. The tone and flow of the conversation also changes in different playthroughs so choosing the same options as you did before in the same order may not work so you actually have to pay attention to what the person is saying and respond accordingly. Dialogue choices and non-trivial and failing to persuade people will result in you having to take a more difficult route or may even prevent you from doing certain sidequests.
The augmentations are implemented well. You really feel upgraded at the end of the game compared to how you started and the option to pick any augmentation at any time is actually pretty cool. Simplicity is often better and I like that nothing is withheld until the end game. They're also pretty well balanced. They all give significant advantages and none of the upgrades are useless. In fact, pretty much nothing in this game is useless. All of the weapons are very powerful when used and upgraded properly. The pistol and be upgraded to ignore all armour (while being silenced) and the revolver can be upgraded with exploding rounds. Nothing in the game becomes obsolete, you just have to choose what to focus on. Weapons take up a significant portion of your inventory and ammo takes up space too so choosing to carry and use any weapon is a significant choice so you're best off choosing your favourites and upgrading the hell out of them.
I do think though that
A) Radar should have been an upgrade, rather than given at the start
B) You should have started with an upgrade point or two with Sarif or someone else advising you on what you should buy depending on your playstyle, i.e., armour if you say you want to go in guns blazing or radar if you want to sneak.
So now here are the problems I have with the game:
One problem I do have is the lack of variety in enemies. Standard enemies do come in the augmented variety but the only augs they actually have is damage resistance, stealth and the Icarus landing system in one cutscene. No one has a sprint upgrade, no one uses melee takedown moves on you, no one has the tracking target aug (which would stop you being able to hide in the middle of firefights), no one has the x-ray vision aug (which would certainly put an interesting twist on stealth gameplay), no one has gas immunity (not even bosses) and no one will attempt to un-hack bots and turrets after you reprogrammed them (no one seems to have EMP grenades either) and there are some of the heavy weapons go unused by any enemies for some reason.
Enemies also have that annoying ability to be able to tell the difference between their friends footsteps and my footsteps for no good reason. Even if I'm walking in an area where they should expect to hear footsteps they'll be alerted by mine in particular. But of course moving boxes around they barely notice. It's an annoying AI quirk and I wish someone would make AI that can't distinguish between player noise and other noise to make it make more sense.
I also think the game could have used more sidequests and particularly some sidequests that are impossible to complete with certain playstyles. The main story can be finished using any combination of lethal and non-lethal force (or no force), whether you bought hacking or not etc. but as far as I know there is only one sidequest that requires you to have an aug (level 2 hacking) and none that require you to kill anyone (though there are some that request non-lethal force, which you can ignore and get shouted at, but you still complete the sidequest). I honestly thing there should have been a sidequest that is a straight-up assassination. You have to kill someone and if you don't like it your only other option is to talk to your target and convince them to flee the city, making you fail the mission. I think there should have been one that requires level 4 or 5 hacking and one that required you to be completely stealthy; not being seen, not tripping an alarm and not knocking out or killing a single person or you fail the mission instantly.
Bosses are also a problem and seem to be the most frequent complaint. Yes the original had boss battles and killing one person was mandatory but even then it was possible to kill Anna simply by hacking to find her kill phrase and saying it to her, avoiding the fight completely. Human Revolution offers no such options. Even if they just fleshed out the boss arenas a bit it would have been something like having a hackable security system you could turn on one of them or some way to flood part of the arena with gas (which you can buy immunity to), denying the boss access to it. There is one element like this in one of the boss battles but that's it. There's also a glitch that lets you do an instant takedown on one of the bosses but that's unintentional and completely broken. The bosses are also all pretty dumb. They'll repeatedly run into mines thrown in plain sight and will not adapt to the strategies you are using. They could really have used a lot of work to make them more open in how you're able to defeat them and make them less susceptible to repeating cheap tactics to beat them.
Some stuff towards the end of the game seems a bit unfinished too. There are a few jarring moments and stuff that seems to have been stitched together quickly to finish the game off. I wish they had been able to spend a bit more time polishing this stuff out. If only Valve made these sorts of games!
As a comparison to the original game. The gameplay and general mechanics are much better. Pretty much everything is. The enemy AI, the combat, the stealth mechanics, the conversation system, the inventory system and the weapon upgrade system are better I think. There is some degree of freedom lost and interaction with non-story relent NPCs is less but I think the improvements cover the losses handsomely. The loss of the skill system doesn't really bother me. In the original your effectiveness in combat depended on your skill, your augs and the upgrades on that individual gun and while not being able to effectively use a gun without that skill makes sense but the way accuracy and the targeting reticle worked in Deus Ex was one of the reasons gunplay was so godawful in the first place. I think makings augs (recoil reduction and aim stabilization) and making weapon upgrades more powerful was a better decision. As for the non-combat skills hacking could be transferred to a skill-tree type setup I guess but it wouldn't really make a difference and most of the other skills from Deus Ex were either useless or lockpicking, which Human Revolution drops entirely.
I was a bit sad over the loss of lockpicking at first but then I realised it would just be another skill for the stealth portion of the game and it would be pretty much impossible to make it logical (Deus Ex's lockpicking system made no sense) and as fun as the hacking mini-game. TBH it would have been superfluous and if there is anything Human Revolution did right compared to Deus Ex was dropping superfluous elements and trimming the fat very well. So I'm not actually sad to see lock-picking, or indeed, the entire skill system gone. In the original skills acted like your stats and augs acted like magic spells, which I think wasn't the best way to go about it. Is there any particular reason you should have to activate increased strength after buying it to lift heavy crates? I don't think so and I'm glad Human Revolution made it and most of the augs passive. Augs in Deus Ex are, as I said, magic spells. In Human Revolution they're upgrades, like they should be.
So yeah in conclusion it's a great bloody game.