I first watched Death Note in my early twenties, and it's interesting to see how I experience it differently now that I'm older.
The main difference is that, when I was younger, I thought Light was a genius. I still disagreed completely with his philosophy; my change in perspective on Death Note wasn't a change of moral stance, although I think a lot of fans did go on a journey from 'Light was right' to 'actually, I'm not sure about all this murder' with age. Even if I opposed his actions, though, I thought of him as hugely intelligent. I was intimidated by the prospect of writing fanfiction about someone who seemed so much smarter than me.
Nowadays, I think of Light as a kid. He's a smart kid, but he's a kid, and I think the 'kid' frequently plays a larger role in his decision-making than the 'smart'.
For example: when L deduces from the times of death that Kira is a student, Light responds by killing one criminal every hour for two days. With these very pointed murders, Light is making it clear that he knows the theories being put forward in the investigation, and therefore he's telling L that Kira has access to confidential police information. It seems like a bad move - it narrows down the pool of suspects enormously - but Light claims he did it deliberately to create suspicion between L and the police.
When I was younger, I took Light's 'this is a very smart plan and I'm definitely not shooting myself in the foot' claim at face value. Revisiting Death Note, I strongly suspect that Light, annoyed that he didn't think to obscure times of death earlier, is just making up an excuse to go 'you think you can deduce things about me? fuck you, you can't,' even if he knows that he's endangering himself with that 'fuck you'.
Light is observant, creative, good at reasoning and able to think on his feet, but he's also egotistic and petty. He's persuasive, but that means he's great at persuading himself that whatever he really wants to do just happens to be the correct course of action. I love that, whenever L tests him, Light spends ages thinking 'if I give the correct answer here, will it seem more likely that I'm Kira?' and inevitably ends up giving the correct answer anyway, because, suspicion be damned, he just wants to look smart.
It's also hilarious that, when L criticises a move Kira makes, Light leaps to explain why it was actually exactly the right strategic decision. Not that Light is invested in having people believe that Kira is really smart, of course. But, just so you know, Kira is really smart. If Light were Kira, which he's not, he'd do the same thing.
At first glance, Light Yagami seems more put-together than many of my favourite characters. Under the surface, though, he's a seething mass of flaws, and every one of them delights me. What a great character. I love him, although I very much doubt it's mutual; if he ever came across this website, he would kill me immediately.