It's themed and timely. I'm not a great coder and don't have a CS background, but have found it very useful to stretch my abilities to the limit. Its much more friendly than any code competition site I've been on, abs also it's useful being able to go to reddit to get tips or occasionally help others (very occasionally in my case!).

I look forward to it each year.

The quality of the puzzles. They're 2-stagers, where the first stage is generally to ensure you've understood the problem, and the second is a "twist" like a vastly expanded problem space.

The factor that makes it stand out for me is the community. The r/adventofcode subreddit is very active every year and not only can you learn programming tips and tricks from others but you can also see people doing crazy things like solving problems in tools like Excel, Git, etc. and cool visualizations of their solutions.

My experience of other coding challenges is: here's the problem, and you submit source code which is run against many test cases to confirm correctness. Advent Of Code only asks you to submit an answer, not source code.

https://projecteuler.net has the same concept, but it's more oriented towards math problems.

Nothing fundamentally different. Just a well done one.