Book of Demons is immediately jarring, not just because of its slickly-animated high-definition papercraft presentation (like an amped-up Epistory), but also because it's a clear "homage" to Diablo. And when I say "homage," I mean that there is an old man standing by the tiny, depressed village's well, and he asks you to delve under the haunted church, into the 3/4-isometric catacombs to fight evil.
They're clearly going for a nostalgia angle, but to me it just feels uninspired. At least Torchlight, for all it cribbed from Diablo, tried a little bit to make a game world of its own. Book of Demons puts forth no effort whatsoever.
That said, Book of Demons does propose a suitably unique set of gameplay systems: equipment, spells, and passive skills are all squashed into virtual cards. Your mana is consumed either by a card's base/equip cost, or by using an ability -- so, with six mana points, you could equip a four-mana weapon and cast a one-mana spell twice. (If I understood the system correctly.)
The game also makes an attempt to streamline combat, although I think it is somewhat misguided. There is an auto-attack feature, so you will automatically hit the closest enemy at a slow rate, but if you click and hold on the enemy your attacks will be more rapid. Which makes the auto-attack more of a nuisance than anything, as coming in range of an enemy will just annoy it with slow attacks.
There are some fun twists on click-and-hold combat, like shields that have to be clicked before you can start damaging the enemy underneath, or infected enemies that explode when they die, requiring some tactical movement.
There is also an irritating mechanic where removing poison requires clicking on your health orb at the right time. (My guess, at this point, is that they are designing primarily for mobile/touch-based platforms.)
Dungeon movement is also ... streamlined, in a way. Dungeons are mapped out with grid lines, and when you click on a line, you'll keep moving in that direction until it ends, or until an enemy stops you. It may free the player from having to click-and-hold, or hold a keyboard button, to move; but it makes stopping, or reversing direction, more fiddly and error-prone than it should be.
The last thing I saw in my brief playtime was the game's system for choosing dungeon length. Every time you enter the dungeon, you choose how many floors will be generated, based on how long you want to play. But ... what if you want to go right to a more difficult level, like how Diablo allowed you to quickly travel to deeper floors? It isn't clear to me if the game has a feature like this, or if it wants me to keep playing freshly-generated "early" floors, over and over again.
Book of Demons has a really cool visual style, and there are a couple interesting mechanical riffs on Diablo-like clicking. But its clear efforts to court the mobile-games audience do it more damage than favor, in the form of a shallow narrative, an unnecessary "card" metaphor, and mouse-unfriendly controls.
Progress: Played the demo, finished two short dungeons.