Light Yagami is one of my favourite fictional characters of all time, and I have spent way too long thinking about his internal workings, as evidenced by the existence of this website.
Pride is Light's defining quality. He always thinks he's the most intelligent person in the room, which can lead him to underestimate others; this helps L to narrow down suspects early on, when Light hasn't yet realised how formidable an opponent he's facing. Light quickly becomes frustrated if this view of himself as comfortably above everyone else is challenged: for example, if someone outwits him or puts him on the back foot.
When Light outwits someone else, he likes to gloat about it. This can lead him to take unnecessary risks; for example, he outright tells Naomi that he's Kira in the knowledge that she's about to die.
Light's arrogance is probably the biggest threat to his plan, L excluded. But Light's arrogance is also the reason he came up with his plan in the first place; he aspires to position himself as a god because obviously, as the smartest person in the world, he's the most qualified to decide how everyone lives.
Light is charismatic and, at first glance, well liked by his peers, although I'd say he's actually more respected than liked. He doesn't really have friends, which is largely by choice. He'd probably make friends easily enough if he cared to try, but he is entirely uninterested in other people; everyone else is too far beneath him to be worth his time.
(Side note: while Light isn't really interested in his peers, I do think he genuinely loves his family, as discussed elsewhere on this website. It's not an aspect of him that Death Note really dwells on, but it's a hint of warmth that I think adds an important and interesting dimension to a character who's largely very cold.)
I haven't seen the adaptation itself, but I found the trailer for Netflix's live-action Death Note adaptation extremely offputting because it made Light look shifty. No! Light is supposed to be the clean-cut, respectable teenager who no one would ever suspect! This is why, while I think most people were joking when they said Zac Efron should play Light Yagami in an American adaptation, it's actually kind of perfect casting.
One important thing to bear in mind, in any attempt to analyse Light's character, is that Light is a deeply unreliable narrator; it's one of his most interesting qualities as a protagonist. You can't believe anything this boy says. He lies to others, of course, but he also lies to himself. He wants to build an image of himself as smart and objective and morally justified, and, if any of his own actions or desires conflict with that image, he'll find any excuse to explain them away.
Because of this, Light saying or thinking something doesn't necessarily mean that that thing is true. One notable example comes when Light first sees Ryuk. Light visibly forces himself to stop shaking and says, 'I'm not surprised to see you, Ryuk. In fact, I've been expecting you.' Light, you just screamed and fell off your chair; nobody's fooled. Except perhaps you, because you really, really want to believe you're a calm, collected person.
I often find myself disagreeing with Death Note analysis that takes Light's claims at face value. There's obviously room for interpretation here, but, throughout this website, I'll point out various times I think Light's narration can't be trusted.
I think Light is a terrible person, but I adore him as a character; I find him so much fun to watch. He's just so weird and intense! I love watching him come up with convoluted plots that screw over everyone around him, himself occasionally included. I love watching him firmly convince himself that he's totally, totally justified in the terrible things he does. I love that his ego will never let him resist a challenge or an opportunity to rub his victory in someone's face. I love the tiny hints of fondness for his family we very rarely catch glimpses of, and I love that, even if that softer side exists, it's not enough to stop him from going too far; he's absolutely prepared to bury it if he has to.
I can absolutely understand why people some hate Light; he's awful! But somehow I just find everything about him enchanting.