Back home, I can be a little more verbose now.. (Hah. A little?)
How does your game go about doing that?
Well, in several different ways. One is the Deviation system, where you can get small physical and metaphysical mutations which build over time, in response to use or overuse of magic. (This includes 'panic button' mechanics, where the player, in a tough situation, when their character is not likely to survive, can draw in more power than they would normally be able to, but take on deviations as a form of backlash.) These deviations have a mix of positive and negative effects, with positive favoring the front half of a given deviation progression, and negative favoring the back half, with the difficulty that once you start down a path, your character is going to keep progressing down it whether you as the player want them to or not.
Another is Instinct and the Feral bonus. This is one of those benefits derived from the Deviations, but it also exists outside those bounds as the Rage mechanic. Basically, you have this Feral bonus, a number which you can improve, which adds to a number of places, mainly to your attacks and your Will defense measures, and exacts a toll in energy on your character. Specific circumstances, in part dictated by an Instinct concept, though, will force you the player to test the character's Will against their own Feral bonus. (One example of this is were-creature shapeshifting, which requires you to test your Feral vs. Will to take beast form, and Will vs. Feral to change back to human.) By investing in your Feral bonus to get its boosts for combat, you're also setting your character up to have challenges when you need to oppose your own instincts, requiring a careful balance of your character's abilities.
What is the horror mechanic? Why am I the player scared?
Instilling fear is a very tough concept to mechanically write. Hopefully the challenge inherent in managing the character has a part of this, but a good portion also relies on a good storyteller-as-GM to set up the situation, drawing out the character's details and history to involve them in the overall scenario, using their flaws against them. I know you don't favor it, but this is one of those concepts I can appreciate from the nWoD, the idea that not only are there bigger and badder things
out there than you, but that you might become one of those things
without even realizing it.
It's not something just anyone
can do right off the bat, it takes some fiction-crafting skill and experience. (I'm not saying that only specific people can do
it, but that anyone who wants to try needs to take time to learn the process and skills of writing for fiction.) I know that means that less people are going to be able to run this game than might be said of other systems, but I think the reward is greater if they know what they're doing in that regard. I really do think it's a skill anyone can learn, though, they just need to take the time and have the patience to do so. (On the other hand, that time and patience means a lot of people aren't going to be able to, just because they don't have those aspects themselves.) I'm going to try to break those steps down properly, if I can.
how is your action/sdventure carried out?
It is a modern setting, but my principle focus is on combat scenarios and mechanical systems, since I come from a wargame background. The secondary intent is to have mundane and social situations to interweave the combats, including some underworld politics and mystery situations, but I'm mainly setting up a framework there, to allow individual games to expand on them if they find that aspect more interesting within their games. Maybe future supplements, but I'm not intentionally leaving a gap to be filled later.
Once aspect of these mundane situations is in how these deviations and magical powers affect their every-day lives, with the intent of showing that as they accumulate this power, they need to counterbalance it with their normal life or risk losing themselves to these deviations and instincts. In part, that's reinforced by provided bonuses to these opposed checks based on those mundane connections. (For instance, relationships and allegiances to other characters, PC or NPC, are a specific mechanic which can be invoked to favor or hinder the characters, including when trying to sustain an opposed Instinct roll. This is somewhat of a slimmed-down and mechanized version of the Dresden Files/FATE system's Aspects, which allow the story's path to be more negotiated between player and GM.)
What does it do different from DnD?
Well, a number of things, but I can say that all day and it doesn't mean anything to you. One of my main concepts is breaking down spellcasting to be just as numbers-interplay-based as weapons and attacks. For the most part, I don't have cast-and-done spells, I have spell components which are crafted together to create a 'spell'. I also don't differentiate between casters and non-casters intentionally. Mundane characters are certainly possible, and may even be more viable than mixing fighters and wizards in D&D, or jedi and non-jedi in SW-d20, but the majority of the material is geared towards everyone having some kind of magic, even if it takes different forms.
I'm not intentionally defining 'roles' for party members, in terms of stats and abilities. This is intended to be flexible in terms of play-size, able to be one-on-one, or up to a group of 8 or so. Characters are intended to be stat-based but not stand-dependent, I'd like to try to have it that two characters with very different stat setups should both be viable in the same vague concept, with those stats determining varying playstyle rather than setting one character as inherently better than the other. (How much of that I'll be able to maintain is another matter, but I'm hoping I'll be able to see and fix these problems in the metagame as I start playtesting.)
I'll also add that I'm trying to explore a concept I hear waved around, the idea of the classless, level-less system. I don't agree with a lot of arguments made about it, but I'm exploring what might be a system compromise between level-based and non-leveled systems. My system right now has freeform advancement, but the most significant form of that advancement is by purchasing levels in 'archetypes', my classes, except instead of locking off one whole character concept per archetype, they just build towards one mechanical concept. So, for instance of my four 'base' archetypes, one's quick & accurate, another's strong and tough, another is smart and flexible, and the last is the 'support/reinforcement' type. Because it's a classless system, though, everyone could take levels in these classes, to different benefits. For that matter, because I'm currently structuring XP to be such a fluid, fast-moving resource, you could buy a level in one of these archetypes mid-combat. Bump a couple numbers, add the level's benefit, throw some skill ranks on the pile (or don't, in one case), and you're good to go.
most important: what about the mechanics would make me choose this game over other aa games?
Honestly? Not sure, in a purely mechanical sense, since I can't determine what others will like or dislike ahead of time, only react to what preferences they show once they've shown them. All I can really do from the outset is make a game I think I
would enjoy playing, and hope enough people play like me to also find it enjoyable. Part of that, for my case, is the setting, and how well it's integrated into the mechanics. This started as houserules on top of the nWoD mortals rules, and then as an overlay for d20 modern, then back to the nWoD. Neither really worked, since I was fighting too much the existing problems and systems which clashed with what I wanted out of the game I wanted to play.
I'm not aiming this as a purely mechanical package for presentation, however. I'm developing this system as a mechanical side of a media concept I'm putting together. I'm actually a 2D animator by trade and degree, and I've been working on this world-concept as the subject of my projects in animation classes for the past four years, and as my own fictional world I've written for a decade now. I've been wanting to develop this story into an animated web-series for a while, and that's still my goal, although I've settled in the short term on developing it into a webcomic in my spare time, until I can get enough funding and following to be able to support the actual series. The RPG is a supporting effort in that regard, incorporating more of my interests into the whole. (Y'know how they say your dream job is doing the exact same stuff you'd be doing anyway, if you didn't need to work? I'm working on creating that. I love animation, I love roleplaying games, and through the supporting aspects of bringing both to life, I've been working on what I want to do for the rest of my life. It'll just take time to get there.)