My impressions from 3 hours of play:
They definitely ripped off both Assassin's Creed (climbing, stealth take downs from bushes, climbing towers to unlock areas) and the Arkham series (combat, upgrades, a "heightened awareness mode"). However, they equaled if not improved upon those mechanics.
The game is VERY combat-centric, so it's fortunate the fighting feels so good. Countering is instant, and you'll never have to wait for a previous animation to finish before you can counter. I can't tell you enough how much I love that, as it's always been a gripe of mine in these types of combat engines. You feel like a kick-ass warrior as you stroll confidently into camp taking on 5 or 6 orcs at a time right from the get-go - however, the game is also quick to remind you how vulnerable you are. Before long you'll end up attracting the attention of too many captains, or slip up some counters from being outnumbered, and you'll realize that you can only take a handful of hits before going down. It's an amazing balance between feeling powerful while being acutely aware of your limitations.
As leo pointed out, the game relies on you to do some story missions early on to really explain the mechanics. If you choose to ignore this and do your own thing (which is certainly a viable option), then just be prepared to be plopped down into a world full of stuff to do almost immediately. Unlike many open world games that ease you into areas little by little, you have a pretty large area in which you can roam right from the start.
The Nemesis system is really neat, and you do get a sense that the Uruk leaders are relative bad-asses among their "people." I really liked the Intel system which requires you to seek out Intel in the form of informants you beat up and interrogate or plans you find lying around. This Intel can be used to find the weaknesses and vulnerabilities of the captains, which you can then use to kill them quicker.
As far as customizing your character, you have the ability to both power up your weapons with runes, or power yourself up with ability points. You can also purchase traits (not sure what they are called in game) that boost health or unlock new rune slots. While the upgrade paths are fairly limited and linear, the fact that there is so much "stuff" to unlock and upgrade really gives you a sense of purpose when killing enemies. The UI provides constant feedback on XP and currency gains, so you always feel like you're making progress.
One other thing I really appreciated is how well the appendix is laid out. Nothing drives me more crazy than a badly implemented catalog or appendix in which you can't tell what entries are new, or which ones have already been read. This game does a great job of showing you not only what you've read and haven't read, but how many entries you have left to discover. Entries are detailed but not overly long, and I enjoyed reading them and fleshing out the backstory and Tolkien lore.
If you don't like the Ubisoft: The Game structure (i.e. open world with main missions/side missions/collectibles), then this may not be your cup of tea - it contains a lot of those elements. However, it's so well implemented and has so many little features that have not been seen before that I think if you have even the slightest interest in that type of game then you should give it a chance.
Ultimately I think it's the most mature and "adult" Lord of the Rings games we've seen to date. It's violent, it's dark, it's combat-heavy, and it doesn't hold your hand. If you want to march into a camp and take on three captains above your level at the same time, it's not going to stop you. It feels like a game made for people who have had experience with open world games and want something that kicks things up a notch. In terms of a "game features tailor-made for YellowKing list" this one marks off a LOT of checkboxes.
This ain't The Hobbit cartoon.
This ain't Gandalf sitting around blowing smoke rings.
This is a bad-ass undead ranger fucking shit up deep in the heart of Mordor. If you're ready to put down your Legolas dolls, put on your big boy pants, and go toe to toe with Sauron's black army, then come on.