Shadow of Mordor is, at its essence, Assassin's Creed with Orcs. That's not a bad thing at all, mind you; the LOTR franchise is rich in its stories and its histories.
SoM gives us a glimpse of what open worlds will be like on next-gem systems. The story takes place over two "maps" of almost equal size, neither of which is particularly massive – but offer enough to do to keep you entertained. Like Assassin's Creed and Far Cry, you have strongholds to liberate and towers to scale to unlock the missions within the vicinity. Strongholds can't truly be liberated though, meaning that there are enough Orcs for you to slaughter at any given point in time.
And slaughtering is what SoM excels at. You have abilities in both human and wraith form, and your ability to switch between the two on the fly makes for some interesting combos in battle. It's also possible to unlock all the skills with a single play through, which some players may find to be welcome.
When you're not slaughtering, you can rescue hostages, collect runes from the past, and engage in weapon-specific quests, most of which involve more slaughtering. I honestly have lost count of how many Orcs I have brutally butchered, but it's all in the name of fun.
Unfortunately all this murder without much context undermines the gameplay. The story starts with a bang but falls flat until you finish a few missions and concludes in a rather disappointing finale. The story could have been fleshed out so much more, but the game's focus is mostly on combat.
Again, it's a lot of fun, but there are only enough times you would kill war chiefs before you get bored. It's a great combat system in a beautifully realised world with little to push you forward in terms of story other than your desire to kill the next bigger beast.