Ultimate 747
The first thing that becomes immediatley noticable about Loki is his sheer size. While physically he 'only' stands at roughly seven feet (give or take his hair), there is such a solid physicality about him to make him seem a great deal larger. Everthing about him seems incredibly real in a way that puts most of his surroundings to shame. It's nothing necessarily so trite as anything definable, however; and while Loki is certainly fairly solidly built -- albeit in a long, lean sort of way -- this isn't the source of his demanding presence.
Loki's species is also a matter of debate; while he looks outwardly like some kind of demon, or even anthropomorphised dragon (both are close, but neither are true), he is in fact the 'last' known member of a mostly exitinct race known as the jötnar. Above the waist, he looks more-or-less humanoid, with long, tapered ears and short goatish horns. His fingers are topped by thick, blunt claws, and a fringe of coal-ash fur runs down the outside of his forearms. Below the waist, he takes on a much more anthropomorphic appearance, with strong, muscular digitigrade legs ending in large, three-toed feet finished off by primitive, hoof-like claws. A long, thick draconic tail waves behind him. Like his arms, the end third of Loki's tail and the back and inside of his legs are fringed by silvery fur.
Loki's body is a mess, and is criss-crossed by old scars, the vast majority of which span across his back, shoulders and legs. He has a strange, cross-hatched mass of scar tissue on his left forearm (of all his injuries, it is the one that looks the most deliberate). He also has three strange bands of concentrated scarring, horizontally bisecting the front of his body across his chest, hips and 'ankles'. He is missing a nipple as a result, and the one remaining (the left) is pierced. Underneath the scars, Loki's skin is an extremely unhealthy mix of 'human' colour tinged with mauve; the whole effect gives him a rather corpse-like look, and generally not in the romantic, pale-vampire kind of way. The exact shade of his skin also appears changable; the mauve tints come to the fore especially if he has been exerting his abilities, or when he is otherwise particularly engaged with his environment. Interestingly, the more 'mauve' Loki gets the healthier he appears, hinting that this shade may, in fact, be his natural skintone.
If Loki's body is a mess his face is even worse. Blank milky-green eyes stare blindly out from eyesockets that have seemingly been burnt purple-black by some past abuse, tear-track like streaks cutting diagonally across prominent cheekbones. A wide, fanged mouth is framed by corpse-black lips, apparently held together by two leather thongs at either corner. While disconcerting, these apparent stitches do not impeede Loki's ability to speak or eat, and indeed appear to stretch and contract like tendons with the moving of his mouth. Nevertheless, they do present a somewhat terrifying visage for those not used to the sight. A long, pointed tongue lives inside a purple-black mouth. He currently has long, wild silvery hair that has a habit of arranging itself into smooth, loose dreadlocks. Interestingly, Loki seems to think his hair is (or should be) fiery orange; he seems constantly surprised to see that it isn't.
Loki's interior physiology is also somewhat unusual. Insofar as it can be established, he neither breathes, sleeps nor eats; his metabolism is so slow that it can almost rightfully be described as non-existant. Due to this he is not 'alive' in the usual sense of the word. Conventional poisons and narcotics have almost no effect on him whatsoever, and he can sustain massive physical damage without 'dying' (truly massive physical damage will send him into a state of non-functional unconscious). Loki also paradoxically possesses a somewhat accelerated healing process. Loki has no heartbeat, and thus does not 'bleed' in the conventional sense, however he still possesses deep purple blood, which qiuckly oxidizes to an iridescent green-black in contact with the air; when in this state it is highly caustic. In all contexts Loki's blood is highly poisonous, containing a mixture of toxins that effect the nervous (loss of muscle control) and cardiac systems (lowered blood pressure) in a way that is very similar to an extremely concentrated dose of snake venom. Loki's other bodily fluids contain similar properties, though not in anywhere near fatal or harmful doses (helpfully, Loki's saliva is a mild local anesthetic, and the 'tingling numbness' it causes is often perceived as plesant).
When he moves, Loki is strong, fast and agile. Interestingly, he is also immune to fire. Note that this only extends to the actual fire itself; he can still be burnt or injured by, for example, heated metal.